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The basic risks in the Thames are temperature, pollution, current and boats.
Temperature is manageable with acclimatisation and/or neoprene.
Pollution is an issue as the sewers frequently discharge into the river, mostly after heavy rain. The water is fairly safe a few days after a discharge as the sewage gets flushed out to sea. This will be better, but not fixed, when the Thames Tideway Tunnel opens.
Current is more tricky. You are okay at slack tide but the pull even 20 minutes after the tide turns is quite something. You need to plan where you are going to get in and out carefully. Tide tables are only approximate due to weather influences so you really need some local experience and knowledge or a guide.
Boats are a problem. A swimmer in the water is tiny and difficult to spot and some of the boats go quite quickly. You basically need a roped off area or an escort boat.
The legal position is swimming in the Thames between Putney Bridge and Crossness requires permission of the PLA. As far as I can tell they always say no, even to people that have things like support boats and have done similar things in the past.
jellybaby-
Thanks. Other things the latest article mentioned were specific to jumping from height, which I'm sure is not something that even Thames swimming advocates would recommend:
“Jumping from a height into the river brings all kinds of hazards. The Thames is a very fast-flowing river, running twice as fast as an Olympic swimmer can swim, so anyone jumping in will be swiftly swept away.
“There is also the danger of hitting hidden obstacles, particularly close to shore. A tidal range of seven metres means there’s a huge difference in the depth of the water between high and low tide.”
Even if a jumper didn’t hit an obstacle, the spokesman added, jumping from height can leave people severely winded and there is a danger of suffering cold-water shock which can result in drowning.
I'm sure that different water levels can be a problem even when not jumping, or indeed objects underwater (such as those kl mentions exist in Shadwell Lido, but which in this case again would probably mainly affect people jumping in).
Oliver Schick
I do share that general attitude, but it didn't occur to me in the context of the Thames. It would be interesting to get a full run-d0wn of the risks--in news reports, you only ever hear a few that people use to make the general point, but I'm sure there must be more. I personally can't understand why anyone would want to swim in the Thames in London, but what I've always wondered about is why, if the river is now supposed to be cleaner than it was, there isn't one of those lido platforms that you get in other European cities. I mean, Livingstone wanted to copy the Paris Plage, but without people actually being able to swim in the Thames, but there are plenty of river swimming facilities elsewhere. Perhaps there are some constraints that really can't be overcome so easily.