We were at DC for President's Day Week 2 years ago. It was plenty warm. When were were there this year in March, it was much colder (only in the mid-60s) and the wetsuits helped a lot, but getting out of the water was the problem!
As Gina said, the freshwater oasis is much warmer than most pools, and if it is really cold out (or even if it isn't) wait to get in your suit and go to the aviary first. First thing in the morning, the birds are a lot more active, and within a half hour of opening, they've been fed and aren't as active.