Jaws, or Giant Piranha, is a boss-level enemy that only appears in the rushing water level.
Size: Very Large (3x2)
Special: Dies if taken out of water.
Though there hasn't been any public indication of his official name, in the source code, he's known as Jaws of the phylum Megamouth. However, his obvious resemblance to normal piranha, as well as being escorted by a large number of them, leads most to think of him as a giant piranha.
The Giant Piranha is the Area 2 mini-boss that you don't want to fight without being prepared. In fact, you might not even know it exists, due to a combination of the moderate chance of the rushing water level appearing, and that you will usually never have to go into the water to complete the rushing water level. Simply reaching the piranha is an ordeal in and of itself - the water it resides in is filled with swarms of the smaller Piranha, which might kill you long before the Giant Piranha decides to chase you. The boss does drop 4 supply crates, which tend to carry Ropes and Bombs, but you may always get lucky and find something very useful. On the off chance you do decide to kill it, the Piranha is virtually impossible to whip without taking heavy amounts of damage. The best way to kill it is to use either of the guns, Shotgun or Pistol (note that the Giant Piranha can take TWO point-blank shotgun blasts before dying), or just throwing a Sticky Bomb into its maw and swimming far away from the blast radius.
There is a method for relatively safely defeating a Giant Piranha with just your whip and no other items. It can be safe and reliable for expert players. First, clear a screen's worth of piranhas. You'll need an empty area to work. This is the riskiest part as well, so be careful not to take damage from the piranhas. Second, lure the Giant Piranha into that section of the screen. Get over or under him (best done by hopping out of the water for a moment which causes him to freeze). Position your self above or below his center and he will turn in place helplessly. From there, swim up INTO his body (only his head will hurt you) and whip furiously. This method is extremely reliable when perfected, but may take a few failures at first to learn.