If you check the previous page, someone posted last year’s weather from March, so that should give you some idea. It’s super variable, but usually most days hit a nice midpoint where it’s warm enough but not sweltering. (I grew up in FL with a March birthday, my parents never let me have a pool party growing up because the weather was too variable for that. But now I live in Canada and our family is of the “it’s above freezing and the sun is out, the pool sounds good!” philosophy, ha!)
Hard to know for sure what to expect with crowds, but I think the time your going isn’t one of the major spring break weeks, although there will certainly be plenty of people there. As far as I can tell, the major spring break rushes are forecast to be right around Easter, and then that second week of March with Texas, much of Canada, and some other places on break, and I think a lot of the FL schools are out the 3rd week of March.