This is a Southern Stingray (Dasyatis americana), accompanied by a Blue Tang as a travelling companion.  These rays forage about in the sand flats and, because they are such messy eaters, usually have one or two fish along with them to feed of the scraps.  Frequently they bury themselves in the sand with only their eyes and gill openings exposed:

When camouflaged this way, they can be very easy to miss.