What makes water clear or murky?

Underwater visibility comes down to how much light is scattered and absorbed by particles in the water. The main things that reduce clarity are sediment stirred up by waves and current, muddy runoff and river plumes after rain, and algae blooms. A calm surface can still sit over murky water for a day or two after a big swell.

What is considered good visibility for diving?

It varies a lot by region, season, and personal expectations, but divers commonly describe under about 3 metres (10 feet) as poor, roughly 6 to 15 metres (20 to 50 feet) as good, and beyond about 30 metres (100 feet) as exceptional.

How can water clarity be forecast?

Clarity can be estimated by combining satellite ocean-color measurements with the physics of how light travels through water, then adjusting for waves, runoff, and algae. Because conditions change quickly, a useful forecast reports a range rather than a single exact number.

What is the difference between a nowcast and a forecast?

A nowcast estimates conditions right now from the latest available data, while a forecast predicts them hours or days ahead. For fast-changing signals like water clarity, a fresh nowcast is often more useful than an older reading.

Glossary

The ocean conditions glossary

Plain-language definitions of the ocean, weather, diving, and fishing terms that decide a day on the water — from viz and Secchi depth to swell period, tides, and algal blooms. Written for spearos, divers, anglers, and boaters.

77 terms across 7 topics

Water clarity & visibility

Water clarity #: How far light travels through water before scattering and absorption make objects fade from view. It sets how far you can see underwater on a given day and is the single biggest factor in whether a dive is worth the trip. See the water-clarity forecast →
Visibility viz #: Diver shorthand for how far you can see horizontally underwater, usually given in feet or metres. “Ten feet of viz” means objects roughly ten feet away start to disappear. Water-clarity forecast →
Horizontal visibility #: The sideways sight distance a diver actually experiences, as opposed to how deep you can see looking down from the surface. It is what divers mean by “viz,” and it is what a clarity forecast should predict.
Secchi depth #: A long-standing clarity measurement: the depth at which a white disk lowered into the water disappears from sight. A greater Secchi depth means clearer water.
Turbidity #: A measure of how cloudy water is because of suspended particles such as sediment or plankton. Higher turbidity means lower visibility.
Beam attenuation beam-c #: How quickly a straight beam of light dims as it passes through water, measured per metre. It is the most direct physical measure of underwater clarity and the closest thing to ground truth for horizontal visibility.
Diffuse attenuation Kd, Kd490 #: How fast sunlight fades with depth, often measured at a blue-green wavelength (Kd490). It is a widely available satellite indicator of water clarity, though it describes light fading downward rather than a diver's sideways view.
Chlorophyll-a #: The green pigment in phytoplankton. Because it can be sensed from satellites, it is a common proxy for how much algae is in the water — and more algae usually means lower clarity.
Phytoplankton #: Microscopic drifting marine plants that form the base of the ocean food web. Dense growths of phytoplankton are one of the main things that cloud the water and cut visibility.
Algal bloom #: A rapid increase in algae or phytoplankton that can turn water green or brown and sharply reduce visibility, even on calm, sunny days with no rain.
Harmful algal bloom HAB #: An algal bloom dense or toxic enough to affect water quality, marine life, or people. Beyond the health considerations, a HAB typically collapses underwater visibility.
Red tide #: A dense, often reddish algal bloom that can drop visibility to near zero and, in some regions, affect air and water quality along the coast.
Colored dissolved organic matter CDOM #: Dissolved organic material, often washed in from rivers and decaying plants, that stains water a tea-brown color and lowers clarity by absorbing light.
Gin-clear #: Informal term for exceptionally clear water — the best conditions a reef or offshore site offers, when you can see well beyond arm's reach.
Blown-out #: Conditions — usually big swell or muddy runoff — that make a spot unfishable or undiveable for the day.
River plume runoff #: A tongue of muddy freshwater pushed out over the coast after heavy rain. It can wreck visibility for days, even when the ocean surface looks calm and clear.
Sediment resuspension #: Waves or currents lifting settled bottom sediment back into the water column, clouding otherwise clear water. Shallow, sandy or silty bottoms cloud most easily.
Wave stir #: Wave motion reaching the seabed and stirring loose sediment into suspension. It is a leading cause of poor nearshore visibility, which is why a calm-looking surface can still sit over murky water after a swell.
Upwelling #: Wind-driven rise of cold, nutrient-rich deep water toward the coast. It can feed algal blooms that lower clarity a few days later, so the clearest water often follows a lull in upwelling winds.
Thermocline #: A depth layer where water temperature changes sharply. It can separate clear water from murky water, so visibility — and comfort — can differ noticeably above and below it.
Oligotrophic water #: Nutrient-poor, low-algae water such as open-ocean and tropical gyres. It is typically the clearest natural seawater on Earth.
Euphotic zone #: The sunlit upper layer of the ocean where enough light reaches for plants to grow. Its depth is a rough guide to how clear the water is.

Waves & swell

Swell #: Organized wave energy that has traveled out from distant weather. It drives the surge a diver feels underwater and shapes how safe a shore entry will be.
Swell period #: The seconds between wave crests. Long-period swell carries far more energy — and stirs far more bottom sediment — than short wind-chop of the same height, which is why period matters as much as height.
Groundswell #: Long-period swell generated by distant storms. It is powerful and, on entry, more dangerous than locally generated wind waves.
Significant wave height #: The average height of the largest one-third of waves — the standard way wave height is reported in marine forecasts, and a close match to what an observer would call the wave height.
Chop #: Short, steep, locally wind-driven waves that make boat rides and surface swims rough and uncomfortable.
Wind wave #: A wave generated by local wind, as opposed to swell that has traveled in from far away. Wind waves are shorter and steeper than swell of the same height.
Surge #: The back-and-forth water movement a diver feels underwater as waves pass overhead. Driven by swell, it is a real comfort and safety factor, especially near rocks and in shallow water.
Sea state #: A general description of how rough the ocean surface is, combining wave height, period, and wind.
Small-craft advisory #: A marine warning that sustained winds or seas are hazardous to small boats. It is a signal to reconsider heading out in anything modest.
Beaufort scale #: A scale relating wind speed to observable sea conditions, running from flat calm up to hurricane force.

Tides & timing

Tide stage #: Where the tide is in its cycle — rising, falling, or slack. Tide stage is a well-known trigger for fish feeding and affects entries and current. How tide feeds the bite score →
Tidal range #: The vertical difference between high and low tide. A larger range moves more water — and more bait — through a spot.
Slack tide #: The brief calm at high or low water when current is minimal. It is often favored for easy entries and settled, clearer water.
Spring tide #: The larger tidal range that occurs around the new and full moons, bringing stronger currents. Unrelated to the season.
Neap tide #: The smaller tidal range around the first- and last-quarter moons, bringing weaker currents and often clearer water.
Semidiurnal tide #: A tidal pattern with two nearly equal high tides and two low tides each day — the most common type on many coastlines.
Solunar solunar windows #: The idea that fish feeding peaks track the position of the sun and moon, producing daily “major” and “minor” windows. It is a common planning input, though the supporting evidence is mixed, so it is best weighed alongside tide and weather rather than on its own.

Weather & water

Barometric pressure #: Atmospheric pressure. A falling barometer ahead of an approaching front is a classic pre-storm feeding trigger for many species.
Sea-surface temperature SST #: The temperature of the water at the surface. It drives fish behavior and location, diver comfort, and how much exposure protection you need.
Marine heatwave #: A prolonged spell of unusually warm ocean temperatures. It can stress marine life and raise the odds of algal blooms that lower clarity.
Current #: The steady horizontal movement of water. Current moves bait and shapes where fish hold, but strong current also makes diving harder and can carry murk into a clear spot.
Longshore current #: A current running parallel to the beach, created by waves arriving at an angle. It can push a diver down the shoreline and transport suspended sediment along the coast.

Diving & spearfishing

Spearo #: A spearfisher — a diver who hunts fish on a single breath with a speargun or polespear. For a spearo, the day usually comes down to one question: can I see the fish? Submarius for spearos →
Spearfishing #: Hunting fish underwater, usually on a single breath, with a speargun or polespear. Because it is sight-based, water clarity largely decides whether conditions are worth the effort. Spearfishing conditions →
Freediving #: Breath-hold diving without scuba tanks. Surface chop, current, and water temperature all shape a freediving session.
Scuba self-contained underwater breathing apparatus #: Diving with tanks of breathing gas, which allows much longer time at depth than a single breath.
Bottom time #: How long a diver spends underwater at depth. It is limited by cold, air supply, or, for freedivers, a single breath.
Kelp beds #: Underwater kelp forests that shelter fish and invertebrates. They are popular dive and spearfishing habitat and have their own seasonal clarity patterns.
Structure #: Underwater features such as reefs, wrecks, ledges, and drop-offs that concentrate fish and give divers something to work.
Reef #: A rocky or coral ridge that shelters marine life. Reefs are prime dive and fishing habitat and often hold the clearest nearshore water.
Wreck #: A sunken vessel or structure that becomes an artificial reef, drawing marine life and divers.
Drop-off #: A steep change in depth where fish often stage. Drop-offs are productive both for divers and for anglers working the edge.
Pinnacle #: An isolated underwater peak rising from the surrounding bottom. Pinnacles concentrate current and fish.
Ledge #: An underwater rock shelf that provides shelter and holds fish.
Flats #: Shallow, often sandy or grassy nearshore areas fished for species that feed in skinny water.
Blue water #: The clear, deep, open ocean away from the bottom, where divers drift for fast pelagic species rather than working fixed structure.
Pelagic #: Living in the open water column rather than near the bottom. Pelagic species — such as tuna and mackerel — are fast-moving and range widely, so clarity in open water, not just on the reef, matters.
Rip line #: A visible line where moving or converging water meets, often marked by foam or debris. Rip lines concentrate bait and the predators that follow it.
Shore entry #: Getting into the water from land rather than a boat. Swell and surge decide whether a shore entry is easy or genuinely dangerous.
Honey hole #: A closely guarded personal fishing or dive spot. Divers keep exact coordinates private, which is why sharing conditions should never expose a precise location.
Spot burning #: Publicly revealing someone's private fishing or dive spot. It is a serious breach of etiquette in the spearfishing and angling communities.

Fishing & regulations

Bag limit #: The maximum number of a species an angler may legally keep in a set period. Limits vary widely by location and change over time, so always check the current local rules before you keep a fish.
Slot limit #: A legal size window a fish must fall within to be kept, protecting both juveniles and large breeders. The exact window varies by location and species.
Size limit #: A minimum — or sometimes maximum — length a fish must meet to be legally kept. Specifics vary by location and change over time.
Closed season closure #: A period or area where taking a species is prohibited to protect it, often during spawning. Dates and boundaries vary by location, so check before you go.
Marine protected area MPA #: An ocean zone with restrictions on fishing or other activities to conserve habitat. Rules range from limited-take to fully off-limits, and boundaries vary by location.
Catch and release #: Returning a caught fish to the water alive, whether by choice or because regulations require it.
Bycatch #: Fish or other animals caught unintentionally while targeting a different species.

How Submarius reads conditions

Bite score #: Submarius's plain-language rating of how favorable conditions are for fish to feed, combining tide, moon, weather, and water signals into one hourly number — with the reasons behind it shown, not hidden. How the bite score works →
Conditions verdict #: A single, activity-specific rating that tells you when to go, with every underlying factor visible so you can check the reasoning. The goal is a verdict you can trust rather than a wall of raw numbers.
Water-clarity forecast #: Submarius's prediction of how far you will be able to see underwater at a spot. It starts from ocean-optics physics applied to satellite and sensor data, then applies a bounded, honest machine-learning correction — and it shows uncertainty instead of false precision. Explore the clarity model →
Best viz window #: The upcoming stretch of hours with the highest predicted underwater visibility at a spot, so you can plan a dive around the clearest water rather than guessing.
Nowcast #: An estimate of conditions right now, as opposed to a forecast of the future. Clarity changes fast, so a fresh nowcast matters more than a day-old reading.
Uncertainty band #: A low-to-high range shown alongside a prediction instead of a single false-precision number, so you can see how confident the estimate is. Sparse data or bloom risk widens the range rather than hiding it.
Activity mode #: A setting for spearfishing, fishing, scuba, freediving, boating, and more that re-weights the conditions verdict around the signals that matter most for how you use the water.

Frequently asked questions

What makes water clear or murky?: Underwater visibility comes down to how much light is scattered and absorbed by particles in the water. The main things that reduce clarity are sediment stirred up by waves and current, muddy runoff and river plumes after rain, and algae blooms. A calm surface can still sit over murky water for a day or two after a big swell.
What is considered good visibility for diving?: It varies a lot by region, season, and personal expectations, but divers commonly describe under about 3 metres (10 feet) as poor, roughly 6 to 15 metres (20 to 50 feet) as good, and beyond about 30 metres (100 feet) as exceptional.
How can water clarity be forecast?: Clarity can be estimated by combining satellite ocean-color measurements with the physics of how light travels through water, then adjusting for waves, runoff, and algae. Because conditions change quickly, a useful forecast reports a range rather than a single exact number.
What is the difference between a nowcast and a forecast?: A nowcast estimates conditions right now from the latest available data, while a forecast predicts them hours or days ahead. For fast-changing signals like water clarity, a fresh nowcast is often more useful than an older reading.

See these conditions for your spot

Submarius turns tides, swell, weather, and satellite data into a water-clarity forecast and a conditions verdict you can trust — with every factor shown.

Open the app → How the clarity forecast works Reading diving conditions