The whole point of wild swimming means that you regularly swim in really cold water, so the temperature of the water in reservoirs, will be familiar to such swimmers.
Regular wild swimmers know better than to swim alone in isolated places, that is left to the kind of idiots who drown.
Properly conducted wild swims are done in groups with strict rules about preparation on site and what your do afterwards and someone on the bank watchng those swimming.
I know nothing about this particular incident, and, to be honest, I am not much interested, I am just concerned about the ignorance about how regular wild swimmers always make sure that they swim safely. the OP refers to all the young people drowning by indiscriminatingly leaping into reservoirs - usually to keep cool in summer. When did you last hear of a wild swimmer drowning?