How bad is it? The title's the best part.
Should you see it? No.
This is a parody sequel to the 2010 Piranha 3D, which was a remake of Joe Dante's quite clever original. There are cameos by Christopher Lloyd, Ving Rhames and David Hasselhoff (who delivers the one good line: "Welcome to rock bottom!") There's also a lot of girls taking their clothes off, though none of the main characters do and the attack scenes are not good, especially given the reported $20,000,000 budget. Piranhas are going from underground lakes to a water park. The tackiest joke involves a piranha in a vagina. The script isn't good, the direction is poor... and there's little else to recommend it.