Open water is a convenient shorthand and a poor category. A tidal sea beach, a muddy estuary, a gravel pit lake and a moving river have almost nothing in common except being outdoors and not being chlorinated. This article separates them, because advice that is correct for one can be irrelevant or wrong for another.
Four environments
| Sea | Estuary | Lake | River | |
|---|---|---|---|---|
| Water movement | Tides, currents, surf | Strong tidal flow, mixing | Little, but wind driven drift | Continuous flow, variable |
| Visibility | Variable, often moderate | Usually poor | Often very poor | Often very poor |
| Buoyancy | Higher, salt water | Between | Lower, fresh water | Lower, fresh water |
| Temperature behaviour | Slow to change, large mass | Variable | Layers sharply, cold below | Colder than expected, varies with flow |
| Glare | High, wide open aspect | High | High, often no shelter | Variable, tree cover can help |
| Bathing water regime | Many designated sites | Some designated sites | Some designated sites | Few designated sites |
| Exit options | Usually many | Often few, mud | Often few, steep or vegetated | Often few, banks |
Sea
The most likely to be a designated bathing water, the most likely to be lifeguarded in season, and the best documented. Salt water is more buoyant, which most swimmers notice immediately. Temperature changes slowly because of the mass of water, which means it stays cold well into summer and stays relatively warmer into autumn.
The hazards are tidal and current driven and they are the Royal National Lifeboat Institution, water safety's subject rather than ours. For exposure, the sea is the most open aspect of the four: no tree cover, no shelter, wide horizon, and the maximum reflected component. See reflection and the water surface.
Salt has its own effect on skin and hair afterwards, which is ordinary everyday care and is covered in skin after salt water.
Estuaries
Estuaries are the environment most often underestimated, because they look like calm sheltered water and behave like neither the sea nor a river. Tidal flow through a narrowing channel can be strong. Visibility is usually poor because of suspended sediment. Exits can be limited by mud, which is a genuine physical hazard rather than an inconvenience.
For water quality, estuaries receive whatever the river brings down as well as anything from the coast, which is why they are more susceptible to the rainfall driven deterioration described in bathing water classification.
Lakes and still inland water
The environment where organised open water swimming most often happens in Britain, because managed lake venues can provide safety cover, marked courses and facilities. Fresh water is less buoyant than sea water, which surprises sea swimmers.
The characteristic hazard is thermal layering. Still water can be pleasant at the surface and markedly colder a short distance below, and swimming across a deeper section can put you into that colder layer without warning. Visibility is frequently close to nil. Depth can change abruptly, particularly in flooded workings.
Lakes also have the least shelter and the least shade of any environment other than the open sea, and organised sessions there tend to involve long periods standing around on an exposed shore.
Rivers
Moving water, which changes everything. Flow varies with rainfall and can change substantially within a day. Cold is often greater than expected because the water is moving and recently arrived from somewhere higher. Underwater hazards are unseen and unpredictable. Few rivers are designated bathing waters, so for most river swimming there is no water quality data at all.
River swimming has grown considerably and the water quality debate around it is public and ongoing. Our position is what it is everywhere: check the regulator, understand that an undesignated site has no data rather than good data, and treat physical safety as a separate question with the Royal National Lifeboat Institution, water safety as the reference.
Currents, tides, rip currents, weirs, flow, underwater obstructions and cold water shock are all serious and all covered properly by the Royal National Lifeboat Institution, water safety and by organised open water providers. We are not going to summarise them, because a summary of safety guidance is the worst version of it. Read it at source.
Does the type of water change my sun exposure plan?
Not applicable. Indoors there is nothing to plan for and the water is a treated system rather than an environment.
The one indoor equivalent is that a hall with a roof that opens becomes an outdoor pool, at which point the outdoor plan applies.
Yes, mostly through shelter and duration. The sea and open lakes have no shelter at all and the longest sessions. A tree lined river can offer real shade, which is the only open water environment where that is true.
Duration is the dominant factor everywhere: the session is always longer than the swim, and the standing about afterwards is usually the longest segment.
Choosing which open water environment to use for a session
Owns it entirely for a solo swim, and owns understanding that experience in one environment does not transfer to another. A confident sea swimmer is a beginner in a river.
Owns matching the environment to the group. A managed lake venue with safety cover is a different proposition from a stretch of river, and a leader taking a group somewhere they have not assessed has skipped the central task. Owns the site specific assessment rather than a generic one.
Where a venue operator exists, owns the site: entry and exit points, marked areas, safety cover, signage, facilities and information about the water. Many open water locations have no operator at all.
Where a duty attachesWater quality data from the Environment Agency, Scottish Environment Protection Agency, bathing waters and Natural Resources Wales. Water safety guidance from the Royal National Lifeboat Institution, water safety. The duty to assess risks for an organised activity sits under the Management of Health and Safety at Work Regulations 1999.
The same site changes through the season
One more variable that a comparison table cannot capture: each of these environments is a different place in April from what it is in August. Sea temperature lags the air by months, so the coldest sea water is well after the coldest air and the warmest sea water is well after the warmest air. Rivers rise and fall with rainfall and can be a different watercourse entirely after a wet week. Lakes stratify in settled summer weather and mix again when it turns.
Water quality moves too. Bathing water sampling happens during a defined season, so outside it there is simply less current information, and the rainfall driven deteriorations described in bathing water classification are a seasonal pattern rather than a constant.
The practical consequence for a swimmer or a group is that knowing a site means knowing it across a year, not once. A group that has swum somewhere every summer for five years knows that site. The same group at the same site in March is at a place it has not assessed.
Local knowledge is the source nobody looks up
For every open water site there are people who know it: lifeguards where there are any, harbour and beach staff, a landowner, a local club, other regular swimmers. They know the things no data set holds: where the flow runs, where the exit gets cut off, where the bottom shelves away, what happens on a particular tide, and where people get into difficulty.
Asking is free, works better than any amount of reading, and is the step most often skipped by confident swimmers who have travelled to swim somewhere new. Local knowledge is not a substitute for water safety guidance from the RNLI or for the regulator's water quality data. It is the layer those two cannot provide.
Summary
Four environments, not one. The sea is open, salty, buoyant, slow to warm and the best documented. An estuary is faster and murkier than it looks. A lake layers thermally and hides its depth. A river moves, changes with rain, and usually has no water quality data at all. What transfers between them is your fitness. What does not transfer is your judgement about the site, which has to be rebuilt each time.