Some sharks can stay motionless on the sand (white tips reef sharks, nurse sharks, etc.).
These sharks don't have to swim to bring oxygen to their gills like other sharks (grey, hammerheads tc.)
Turtles are in economy mode when they are sleeping. If they are woken during night, they may not have enough air to return to surface.
So, please be careful during night dives!
Mammals have a horizontal tail.
Fishes have a vertical fin.