Saw this movie back in the late 70s as a matinee double-feature.
Some of the actors faces were familiar but nobody famous and don’t remember any of their names. For a long time I thought the movie was called something like Shark’s Cove or Smugglers’ Reef but that’s probably wrong.
Anyway what I do remember is that most of the film takes place on a sailboat and  in a cove or bay – I don’t remember any open or rough water.
The captain is an older man and in one scene where he’s only wearing Speed bathing suit, one of the other men says to him that he’s in good shape for a man his age.
He replies “I’m in good shape for a man YOUR age” and proceeds to do some difficult pushups with his hands fully extended to the sides and also straight out above his head.
I don’t recall why they were in the cove, possibly looking for treasure or smuggling. At some point some bad guys take over the boat and force one of the men who is either gay or just effeminate to put on a dress and makeup and dance.
He’s also beaten with a belt until he cries.
I hope someone can identify this one; I’ve been trying to find the name of it for many years.