I seem to recall reading on here around the time we got our first snake (sept '07) that one member had a snake that preferred to drag her prey into the water bowl before swallowing it. Don't remember details; I might be confused.

The rat probably did not drown. Constricting would have prevented it from breathing in any water. Or at least constricting supposedly prevents the prey from breathing in and they tighten each time the prey breathes out. What you should have done, for the sake of science, was to take the rat away from your snake and done an autopsy to see how much, if any, water got into the lungs. OK, maybe not really, but it would be interesting to know.