What is the name of the 70's 80's or may be 90's movie that...

There is a cult or gang of rockers or metalheads which have a drug addict long haired leader who lives near a river or lake, and under it there is a secret cave or hidden room under the water that nobody but him knows, and he makes his gang members believe that thanks due to the "father powers" he can stay underwater for more than 15 or 20 minutes holding his breath.

The beginning of the film starts with a young man who is a very good swimmer at an educational center, and then he meets the sect thanks to a girl who was s nice friend, she was cool and also a compulsive smoker, I think she is some kind of hippie.

More details that I barely remember:
The leader wants to form a band but he doesn't know how to play the guitar and when a member who is easily manipulated and obedient manages to get a black flying V guitar, he scolds him saying that what he should get was drugs and not a guitar, days later he ends up breaking the instrument on the floor, precisely in a rehearsal since when trying to play he can't do it... then in the night ritual where they had to be completely drugged, it will be the initiation of the poor manipulable boy, who ends up killing with a knife and hiding the corpse in the hidden chamber that is few meters under the pier... But the athlete begins to doubt his powers of holding his breath and he does not believe that it is really the work of the devil and with the help of his father they discover the secret chamber and apparently they find other corpses in there... I do not remember more.