I have been in the pool (build-in pool at ground level) the last few days and I noticed that there are a lot of dead flying insects in there. Or insects that are about to drown. I rarely swim in natural water and when I do, it usually is not that clear to see if it happens there as well. So are threy drowning in natural water as wel? It’s not that the sides of the pool are to slippery or steep to get out, they sometimes just land in the water and can’t fly back out. Do they drown in natural water? And if not, why do they die in poolwater?
I don’t know for sure, but some thoughts on the matter. If the pool has lights in it, they are attracted to the lights. Also they will land to drink and swimming pools unlike puddles or ponds, usually have steep sides without rocks, overhanging branches, shallow ends etc for them to land on, so they fall in and then can’t get out. Also insects in larger bodies of water that do fall in would get eaten by fish.
I would also guess that pumps play some part to it. While they aren’t that strong, you don’t really need to be strong to win over a fly.