Killer Fish (1979)

A group of jewel thieves blow up a factory to distract authorities from their heist. Since they know police will be searching anyone they meet in the area, the group throws the loot in a reservoir to avoid being caught. Unbeknownst to them, the mastermind behind the jewel heist has filled the lake with piranhas so that no one can sneak back to double cross him and grab the jewels.

In a side plot, a model and her photographer arrive for a photo shoot and end up cavorting with one of the jewel thieves. Through a serious of coincidences, everyone ends up stuck on a marooned ship in the piranha filled reservoir. Close enough to shore to be enticing, terrified enough of the piranhas to think twice about swimming for it. You'd think they'd band together to try to survive, but there's always some jag off who is only in it for himself. Piranha carnage ensues.

I've wanted to see this for a long time since it's about deadly fish and it stars Lee Majors, but it's not very good.  You're much better off watching Piranha.