The animal that was on the picture I posted two days ago, in the bottom of the water, was a very well camouflaged Sand Diver (Synodius intermedius).  This species really blends into its environment.  It has a behavior that helps with its strategy to remain unseen: it stays at the bottom of the water, in a specific spot, that looks very much like its coloration and does not move at all.

Edited to change photo 12/13/15.