Plankton (1994)

It's Gilligan's Island gone berserk, only this time the guys have the good sense to get sexy with their Ginger-and-Maryann equivalents. Five teenagers set sail in a rubber boat, get overpowered by a storm, and happen upon a derelict research vessel. Just as they're getting the party started, these maritime mooks discover that mutant fish have taken over the boat, killed the crew, and are still hungry. What better to do than have the chicks get naked? That's the sort of right thinking on display in this fast, fun, charmingly low-budget horror romp.