@tsarstepan,
A good memory of mine -
my husband and I took off for a four or five day vacation and drove to Death Valley, in, ahem, something like late July. Very very hot, and not a lot of people around. Nice way to die in a car breakdown but it didn't break down, that time, and we had water...
We stayed at the offshoot of the main hotel, a lower end priced motelly sort of place. But, we got to go to the big hotel, Furnace Creek Inn, beautiful building, and use the pool. So we did, late in the evening, and were the only ones in the pool, tre lux. Extremely quiet place at the time.
http://www.dealbase.com/blog/wp-content/uploads/2009/06/furnace-creek-inn.jpg
Finally my husband fessed up - we'd had bats flying over us, I just didn't see them (night blind, also swimming sans glasses). I freaked for a few seconds, then relaxed. Us and the bats.
Searching - they were probably fringed Myotises -
http://desertdeathvalley.weebly.com/animals.html