Piranha II: The Spawning (1981), directed by James Cameron.
What's worse than mutant piranha fish? Flying mutant piranha fish!
I saw this on video tape and thought it might hold up as good cheesy R-rated fun, something like Humanoids from the Deep (1980). It falls short of that. The action scenes are solidly in the cheap effects creature feature genre, but the human interest main story pasting it all together is painfully bad, with weak attempts at comedy.
This obviously owes a lot to Jaws (1975) and the original Piranha (1978). At the time I saw every new horror film through Alien (1979) lenses and I remember thinking "A flying fish erupting from a corpse is just like the chest-burster, isn't it?"
The good aspects:
I recollect Tricia O'Neil as easy on the eyes, but also that they were trying to suggest some other actress: maybe Jacqueline Bisset or Rachel Ward? Adrienne Barbeau?
Lance Henriksen fan club!
The water does not have that crystal clarity we like for really stunning underwater shots, but there are some lovely scenes and exciting, difficult stunt work.
A generous amount of boobage and assorted T&A by lovely women. The opening scene is of ambitious underwater sex. Which doesn't turn out well, given the presence of mutant piranha. That'll teach the kids to try fancy fornicating.
Cameron cites Terminator (1984) as his first film, reluctant to take credit for this one. He certainly didn't have much control over the project and it was a rocky production; see the details in the wikipedia article. Which has many "citation needed" notes, so who knows the complete story?
Available on Blu-ray from Shout Factory.