Dan Nordquist’s review published on Letterboxd:
Enjoyably goofy genre trash. This is over at Trylon as part of animated monsters week (think Harryhausen), but the animation here is sadly as spare as the acting and the writing. The film is more generous with shots of women in (and occasionally out of) swimsuits, splashes of blood-red water, and a burbling electronic sound effect that has to represent the threat of fish that bite. So it's underwater. So you can't really hear them, and they're certainly not going to spend the money to show you. Movies.
Horror films are of course about taboos and mores, and there's nothing terribly insightful brought to the fore here. The US government probably shouldn't have selectively engineered piranhas as a weapon, and if that happened in response to the failures in Vietnam, the film certainly doesn't seem to have a ton to say about it. Nobody should have trespassed in the secret military facility, and our skinny dippers meet the demise we expected they would, but most of the people in the path of the piranhas are the definition of innocent. (The film does take a turn towards exploitation / revenge, but nobody should be surprised when human villains show up in a film called "Piranha", and nobody should be at all surprised when the piranhas eat them.)