I like to swim over the open sand sometimes. Different critters there. I'm fairly sure this is a Spotted Trunkfish, I don't have my books at hand at the moment. These guys go about the sand puffing holes, puff-puff-puff, trying to find whatever that can to eat. There are a lot of worms, crabs, and other things that live on and just below the surface.
Funny fish, their bottom side is flat, and their sides are flat, too. If you cut one in two, whack! with a machete, you'd find that they're shape is triangular.