We were on the 1/19 wonder this year. It was too cold for my liking. Obviously it was better than home, but I didn't swim at all. Temps were low 70s all three days (we were on the 4 day) when we were out, and in the low to mid 50s in Florida. The kids swam, but they didn't lounge around in the suits! Into the pools and out!
The prices were good, but I wouldn't do it again. Most of the fun was being in the water, and it was too cold to enjoy! I'd say March would be perfect, but you'll have to watch out for spring break price increases!