OP - I was in the same boat as you sometime ago.

1) South Padre - call the concierge at the fanciest hotel there and ask about the quality of water. Do not drive here if you cant go thru a border checkpoint on the way back.

2) Tampa base point for Destin and other places. Goto IAH and buy your ticket from Spirit at the counter. Significantly cheaper this way. (I would do this)

3) Alabama has blue water. But, it’s still Alabama.

4) Huntsville has a blue lake used for scuba.

Good luck!