Despite its name, Piranha 3DD did not include any three-breasted women. I'm pretty sure about that because most of the women in the film were topless and had their breasts exposed for all to see, and the others (naturally including all actresses you're likely to recognize from classier movies) wore skimpy bikinis. But getting only two-thirds of the breasts we were promised is far from the biggest problem with the movie.
Chet (David Koechner) and his step-daughter Matty (Danielle Panabaker) are co-owners of a water park that is about to open for the summer. While Matty was away at college learning to be a marine biologist, Chet was busy getting the park ready for their grand opening with touches like the world's first aquatic strip club. He didn't really have a license to do that, but since he was already paying off the local police to overlook some building code violations, that wasn't really a problem for him. Matty wasn't thrilled about Chet's contributions, but there's no time to do anything about it.
The water park is situated right on the edge of Cross Lake, which appears to be separated from Lake Victoria (where the events of Piranha 3D went down), except that they're connected underground. The piranha have discovered this connection, much to the dismay of those who wander into the lake. A farmer was the first to encounter the piranha, but he didn't live long enough to warn anyone else. Neither did the kids whose sexual activities caused their van to roll into the water. But surely there's no way that piranha can get into the water park's swimming pools, right?
As you would expect, Piranha 3DD is simply ridiculous. The plot is stupid, which you probably knew going in, but it also fails at the elements that are supposed to make it enjoyable. There's a lot of nudity, but most of the women on display are so skanky that the piranha that bite them are probably worse off than their victims, and not even the attempts at gross-out comedy are properly executed. The only semblance of horror comes in the form of jump scares and poorly-rendered gore, and the funniest moments come when the movie is making fun of itself or its predecessor.
I only had the option to see the film in 3D, and there I was also disappointed. While it seems to have been shot that way rather than post-converted, the 3D adds absolutely nothing to it. It's really only noticeable when something goes wrong like when you see ghosting of the characters at certain depths, or when a lens flare provides an artificial flat plane in front of you. There aren't any moments when you feel the need to dodge anything coming at you, nor are there breasts so large and three-dimensional that they appear to be poking out of the screen.
There are some things to enjoy from the movie. If you liked Piranha 3D (and why would you see the sequel if you didn't like at least something about the first one?), then you'll probably appreciate the returning characters, and even if you're going in completely blind, there are a couple of fun new cameos. There are a couple of moments of decent comedy, and if you're the kind of person who likes bad movies then there are things about this one that may work for you. But ultimately, unless you're one of those weirdos that actually likes 3D, there's no reason to see this one in the theater when it's also already available through video on demand (and probably for quite a bit less than the price of a theater ticket, especially if you're going to see it with multiple people).