What The? Bat?

I went out to feed my fish and walk my dog the other day and this tiny bat swooped in, hovered over the water, then plopped in face down! He stayed motionless for a bit, then swam to the edge where he preened himself for about 20 minutes and then flew away.

He was so small! For some scale, the slate edge he's hanging onto is about 1 inch tall.