Jack is Alive Underwater. Is he a Fishman?

Jack is shown lying motionless on the ocean floor, waiting for his crew to come and rescue him after being defeated by Zunisha the giant elephant. His mask is broken and his shark-like teeth (resembling Arlong and Hodi – the shark fishmen) is revealed. However, unlike other devil fruit users, he does not pass out or have any difficulty breathing in the ocean.

So, is Jack who happens to be a Zoan type devil fruit user also a fishman, or is there any other possibility as to how he can survive for so long under water?