My guess is this is Bathytosia brevicaudata, only because I go snorkeling in that exact spot reasonably often, and when I check out the stingrays I can see from above like in this sighting they are B. brevicaudata.
Eagle ray (Myliobatidae) of some form. Hard to say exactly what it is without a clear image of the pattern on the back side. Do you remember what the pattern was like?
