You might not need to go very far at all. It was warm enough in early March last year, NE Oklahoma. I was camping at the lake mid-March. Had to give it all up for the swarm of tornadoes the next week. Then went to Michigan where it was still snowing.
It has snowed as late as mid-April in this part of the country, or over 80F highs for a week at a time, so no guarantees.
If I needed to be in a warmer place in March I would go to the Mississippi Gulf Coast, it is the closest with halfway decent beaches. But during Spring Break I expect lots of company there.
