In 1975, Richard Dreyfuss starred in Jaws; 35 years later he had a cameo in Piranha. His connection to the first film is obviously the only reason he’s in the second, wherein he’s killed off before his name even appears in the opening credits.
Dreyfuss is not alone, though; Elisabeth Shue and Ving Rhames also get to whore themselves out. In the latter's case, there was never any danger that Piranha would result in another hand-me-down acting award for Jack Lemmon; on the other hand, Rhames has easily the best scene in the entire movie — using an outboard motor in a way that would make Ash Williams proud.
But I'm getting ahead of myself; Shue and Rhames first have to find the remains of Dreyfuss, then capture a piranha which they take to Christopher Lloyd (in full Doc Brown mode), who identifies it as a species that has been extinct for two million years (according to the subtitles, Lloyd calls it the "regional piranha," but he may actually be saying "original"; either way it doesn’t matter because neither term makes sense).
An earthquake released the piranhas from an underground lake, or some such bullshit. Q: "How could they survive for so long cut off from the surface?" A: "Cannibalism. They must have fought each other to survive.” Until, presumably, there could be only one. Piranha MacLeod? Well, that would actually explain their apparent immortality.
Piranha is supposed to be a comedy — or, specifically, it's a remake of a parody of Jaws released three years after Jaws and 32 before this one. In other words, this material was so diluted it was almost homeopathic.
Shue and Rhames are big fish in a small pond here — kinda like those poor Sea World orcas (I know they’re mammals; you get the idea, though); as such, the only emotion they inspire in the audience is pity over their career choices.
Only Jerry O'Connell, in a role no doubt based on Joe Francis (of Girls Gone Wild infamy), moves as if he's in his own element — because he is; he knows exactly, having been in several others of the same ilk, what kind of movie this is, and navigates it accordingly.