We were there this past March - it was pretty chilly most of the time. We were able to swim the first night, but we didn't go again. A few brave souls were out though.
We wore jackets each morning, and shorts or capris. I brought sweats or leggings for the girls to put on in the evening, although I think they ended up wearing them most of the day. Had a bit of rain here & there. I LOVE that kind of weather though.
Crowds - well, they were worse than I expected. It was ESPN weekend, so MGM was nuts, of course. I'd have avoided it but my dad wanted to see some of the events. AK was also WAY worse than I thought it would be. We did get to the parks early each day so that helped. And the nights were actually OK since it got chilly.
I haven't been in the summer though, so I've never seen a super crowded, level 9-10 day. But these days were certainly full - fastpasses for Soarin', Everest and one other attraction were gone relatively early.