The only bits of the movie I can remember are.

A groups of friends were I think on a large ship or boat.

Some of them get murdered.

One of the young women had her wrists slashed in the bath.

I think it was one of the friends who did it.

Bette Midlers "You gotta have friends" played over the closing credits.

That's all I can remember of it. I do know as a child it gave me nightmares.

Perhaps it might be The Last of Sheila.

Hey thank you so much. This has been on my mind for years. I finally know what the film is called!