That's the final boss, the Vortex Queen, an enormous, screen-filling alien head with teeth bigger than you are. In order to beat her, you have to charge against her jaw until it snaps off -- which is exactly as disturbing in action as it sounds typed out like that. Stop for a second to collect your wits or pray to a God who clearly wasn't looking when this thing was created and you're grabbed by one of the other murky alien creatures that swim from her mouth.
"Squeal like a pigfish! Squeal!"
Beat her and you're rewarded with a sense of unease about the ocean that will linger well into adulthood, leaving you constantly wary of the cold, uncaring immensity of the sea. Plus a credits screen.