This movie is from the 1960s, maybe 1970s.
I think I watched some scenes in the middle and the end.
All I remember is that there is a 13 year old boy who works at a pool and falls in love with a 17/18/19 year old lifeguard named Susan (I think), who already has a boyfriend.
Near the end, there is a problem with the pipes or something. One night, they are both there late and the pool has been drained. The boy lies down in the middle of the pool with something that belongs to the girl (a ring, maybe), and refuses to give it back until the girl undresses. Afterwards, he won't leave however, I forget what happens here, but eventually the pool starts to fill and it must overfill because at some point they are swimming up to the ceiling. The boy gets mad when she attempts to leave and pulls back once of the overhead lights. That gathers momentum and hits Susan, killing her. She slowly sinks and the boy "rescues" the body and the movie ends.
Sorry, that's all I remember. This suddenly fell into my head and I've spent 3 hours searching with no success.