WEIRD! GHOULISH ATOMIC BEASTS WHO LIVE OFF HUMAN BLOOD!
Radioactive waste dumped off the coastline creates mutant monsters. The beasts attack slumber parties, beaches, tourists, and terrorize a waterfront community as a scientist, his daughter, her boyfriend and the local police try to find a way to stop them.
There are a few dancing scenes, but I wouldn’t say that it’s a significant portion of the movie. Maybe you’re thinking of a different beach-party monster movie?