It is called the pacu, and it usually eats insects and fruit/veg. And though it probably didn't need to for that diet, it evolved human-like teeth, which become especially obvious when you cook it; it's finny and funny and oh so delish...
Now they're moving north from the Amazon, and have developed the bad habit of eating people's testicles.
"They bite because they're hungry, and testicles sit nicely in their mouth."...Fishermen in South America attacked by the pacu have reportedly bled to death after losing their testicles in the fish's vicious teeth.
So you'd be nuts to swim around them, but never nut up again afterward. I think it's time to come up with some creatively obscene fishing lures.
UPDATE: CNN is semi-debunking the story. the professor who gave the original quote says the whole ball-eating thing was an exaggeration...but that he's still keeping his swimsuit well tied, just to be safe.