Swimming in tidal waters can be fun if you do your research and follow advice on swimming safely outdoors generally, in rivers and the sea, and especially in tidal rivers and estuaries. And find out about the specific river, ideally from locals who know it well.
Swimming outdoors is not a risk-free activity, but it is possible to mitigate the risks. In addition to the risks of cold water and other hazards, these are the main hazards of swimming in rivers, the sea and in tidal rivers and estuaries (advice in links below):
currents and eddies – which can be faster than you expect, and not always obvious
flotsam and jetsam, hidden obstacles
effects of the weather
health risks and pollution
boats and other river users
tides and rip tides
impact of wind
Tidal rivers and estuaries – all the above plus
silty or muddy water
long gaps between access points
very muddy exit points
changing flow direction
tides and bores
rules applying to navigable waters
There are some advantages
you can use the speed of the tide or bore to help you swim a distance faster
the water can be warmer, as water flows over extensive mudflats
How to swim safely in tidal rivers and estuaries
You need to be aware of all hazards and to understand them, and – very importantly – know the local water and how it behaves. It’s best to tap into local knowledge and to observe the water. Talk to sailors, boaters, canoeists and kayakers, anglers and experienced swimmers.
Learn to read tide tables and sea charts and to understand them. In some places you might want to swim in the slack water either side of high tide, in other places at low tide. Tides are quite complicated, in terms of times, directions and speed. The local tide table might not apply exactly to the place you might be thinking of swimming, and the tide does not always come at the time predicted.
Be aware of boats, keep away from them, and make sure they can see you, with a bright swim hat and tow float. Be aware of the rules of navigation, but adapt to the particular circumstances. Some boaters are inexperienced and can be unpredictable. The ladders at mooring points should only be used with great caution, and not if boats are around, as boats can approach at speed.
Check for sources of pollution. Heavy rainfall can have an impact on water quality. Areas busy with boats can be polluted by fuel or waste spills or discharges.
Heavy rain can mean flooding, faster flowing water, and debris. Be very cautious.
Swimming is banned or only allowed with special permission in some tidal rivers, such as the Thames below Putney Bridge.
More detailed advice
Outdoor Swimming Society advice on rivers, currents and eddies, tides, rip currents, waves, coastal weather, and sea kayakers’ advice on currents.