Indeed... I use the term 'Beach' loosely!For context, it's less of a beach, more a dent in the bank with a bit of gravel.
However, the river was very busy as you say along its length with many reports of poor behaviour, speeding boats, disregard for the rules not that the vast majority know what they are never mind care.
There is a long history of risky conduct on the Thames with youngsters in particular swimming where they ought not to, jumping from lock bullnoses (Teddington barge lock) and bridges, batting about in inflatables, throwing missiles at passing boats, obstructing the fairway.
In the absence of any EA patrol boats and enforcement, it's become a free for all.