It's funny, I could have sworn I'd read rules on drowning in other FATE games (which would then be easy to port to DFRPG), but it turns out I was wrong.
Anyway, I'd go with a fixed number of exchanges equal to Endurance (+1?) before you have to make any checks, then a roll against a fixed, but increasing attack opposed by Endurance every round after that. Perhaps with that increase rate doubling if you're moving around (ie struggling with someone, trying to swim, etc).