This past March was one of the coldest and it was chilly! It was warmer in Colorado where I am from than in Florida. I was able to go swimming just one day and I was still very chilly. It was very windy too. As for crowds, it was leading into Spring Break time and it felt more crowded than in late April/early May or end of September but definitely not as crowded as summer.
My advice - for crowd info, check touringplans (30 to 60 days prior to going if you want the 'free' calendar) and subscribe to tourguide Mike (I used him my last time and it was good advice - spot on for crowds and which park would be the 'best'). For weather, don't get your sights on it being one way or the other just check out accuweather starting 10 days prior to leaving and check everyday on the forecast. By the time you are ready to leave, you should have a good idea on the weather and if you should bring more shorts...or more pants!