Good friends of ours went last year for the 2nd? week of February. They had good weather, cool in the mornings but warmer in the afternoons. They didn't do much swimming, but not sure if that was due to weather or just that they aren't big swimmers.
Unfortunately they ran into President's Day Week-end and they didn't realize how busy it would be. If you could avoid that week/week-end, you'll probably be okay crowd-wise.