Best beach in south/southeast in mid March (lake or ocean)

First off, I imagine the ocean will be cool enough that I think that's out of the question. A little bit of background to start out with. I'm a 25 yo male that'll me going on vacation with with 26 yo female girlfriend. We're very close so I'm not worried about spending a lot of time together or being in close proximity with her. We're looking at going on a 4-5 day vacation together. Most likely we'll be driving. We're from central Iowa but most likely will be taking off from Kansas City, MO. We're looking at a max of 16 hours road time. We're thinking about busting it into a stay at 2 cities for a couple days at a time. I think it'd be fun to power through and find a good lake/beach to crash at for 4-5 days though. My dilemma though is that I don't know any good lake towns to stay at. I want the lake to be warm enough that we'd enjoy our time on the beach and in the water though. Since we're younger I'd prefer a more fun town with plenty to do if possible. We don't want to go to any "spring break" beaches either. Doesn't necessarily need to be south/southeast but I figure that's our best bet. You guys have any recommendations on a good place to go?

TL:DR Looking for a good lake town/beach get away within 16 hour of central Iowa around March 15th