-
There's no cold shock in Shadwell right now - I was in at the weekend, and it's about 19 degrees. I swim there all year, and someone without cold water experience would be in trouble with the cold in January, and probably even in early June, but not now.
In this weather, the local kids go down there for a drink, a smoke and a laugh, which often includes jumping in to cool off. It can be a dangerous combination. My guess is that factors are likely to include lack of swimming ability, booze and drugs, the depth and lack of clarity in the water, and panic.
Yes, the main reason generally given for deaths is that people don't understand how dangerous the currents are in the part of the Thames near Waterloo Bridge. No idea if that's the case at Kingston, too, as I haven't read that many reports about deaths there.
On Shadwell Basin, @kl is best-placed to comment. One problem is often that people feel very hot and jump into surprisingly cold water, which gives them a shock because the heat they feel is only superficial and they're not actually that warm inside. It's in marked contrast to when you've been cycling for hours and get in the sea at Dunwich even when it's freezing--it's an initial shock, but because you're so well warmed-up, it's blissful and quite safe if you don't stay in there for too long.
I'm sure there are other dangers of wild swimming, but I don't know them.