I was on the Dream 2/27 cruise, and on Castaway on Tues. 3/1. It was a warm, sunny day when we were there, and plenty of people were swimming.
We snorkeled and felt comfortable in the water (the water felt a little chilly when I first got in, but I was never cold). Stayed in for about an hour or so, right around noon. At no point did I feel like I had to get out because I was too cold -- it was great!
I don't know if we just got lucky, but it was a beautiful day!!