That really varies.
For my cousins' first cruise (they have been in FL for so much of their lives they might as well be natives) there was a nasty cold snap that mid-February, and they couldn't enjoy any part of Castaway. But then no one could.
My family was in Orlando at the time, and we're from (well, I've been here since I was 17) WA, and my now-ex woudln't even put on long pants until it hit 32 degrees at home, and we were FREEZING.
The following year at the same time of year we all went on a cruise together. The weather was lovely. Beautiful. No cold snap. Just nice typical February weather. And the water felt WONDEFUL. Didn't feel cold at all. Even the little cousins, who ARE florida natives, spent all day in the water.
My family went again that october, and the air was so much warmer that the water, which I'm sure was warmer than it had been in February, felt *colder* to us. Even my son barely spent time in the water b/c the difference between air and water was so extreme.
So it really depends!