As someone who once lived near and knew a friend who worked at a reservoir, and explored the overflow tunnel and even went boating in the reservoir legally, it has to be said: reservoirs are dangerous places. The water temperature can be very cold below the surface, even on a hot day. There can be pipes sucking water out to a treatment works which can make for hidden currents and so on. They are not places for swimming, not at all.

This is a tragedy, and education and signs need to be obeyed. It's like no smoking signs in China, noone even cares....