This is a relatively uncommon scene:  a Nurse Shark (Ginglymostoma cirratum) actually doing something besides lying around sleeping.  Nurse Sharks are a common sight, but usually you find them sleeping on the bottom, often under a ledge or in a sand pit.  The species is unusual in the world of sharks in that they don't have to keep moving constantly in order to get sufficient water flow over their gills for adequate gas exchange.  They are extremely docile and unthreatening.  In fact, when I first learned to dive there was a story going around that if you grabbed a Nurse Shark by the tail and held on tightly, then it couldn't bend around to bite you!  This is nonsense, of course, because like all sharks these guys have no bones--only cartilage.  Thus they can bend around quite easily.