Swimsanity!

If you're high like me, you probably started shooting at the first two lines of enemies that respawn, and thought you had to kill them enough so they would stop and you could progress. This doesn't happen, so shoot them for ten minutes or so on the Deep Blue Sea level. It won't track until after you get game over, which was likely caused from getting scared when you saw the killer whale and panicking yourself to an early grave. It'll unlock when you get to the results screen.

Easiest way to do it is play survivor