This is a peacock flounder, a type of flatfish classified as a left-eye flounder. This is because if you held it vertically upright both eyes would be on the left side. The peacock bit should be self explanatory.
These are fairly common on sand in the Caribbean if you look carefully. Unfortunately this one wouldn't sit still for me at all. This species often changes colour to blend in but this one was keen to show those blue rings as it departed.