Perhaps, if one had been active for hours and goes for a swim during the night, would it be possible for the person to fall asleep or get really sleepy?
In USMC basic training we had to learn the so-called T-Float. It's a survival float where you just hang in the water with your body limp, then every 10 seconds or so you bring your hands up towards your chest, straight out into a T, exhale, and then push down with your hands. You then tilt your head out of the water and inhale. In order to pass, the recruit has to do this for 30 minutes. I did, but fell quite asleep while doing so. They had to wake me up to get me out of the pool. Man, was I relaxed after that!
So yes! You can fall asleep in the water, not necessarily while swimming, as in moving from point A to point B, but you can fall asleep without drowning.