This will be our second year at Gulf State park in Gulf Shores Alabama. Last year it was sunny and low 70's when we were there (3rd week in March). It's green and there are flowers blooming which is what these winter-weary Iowans are looking for.  The pool at the park isn't open yet but there are lots of activities to keep you busy. Love the bike trails. 
We have also camped at Topsail in Destin, Florida that same time of year with about the same kind of weather. Topsail's pool is open but no one swam while we were there. It's not heated. 
Some years it's warm enough to swim in the ocean and some years we just wade.