IMDB isn't an option, because I have no idea who starred in, directed, produced, etc., or even the year it was made.
It's a movie about a boy who goes to a beach house on a (NE?) shore every summer and has a best 'girlfriend' next door neighbor that he hangs out with each year. When they're about 12, they get to the beach, hang out, have a fight one night, and after he storms off, she drowns. When he comes back, he sees her ghost (on the beach or in the water) there. It had to have been released in the 80's, I can't quite see it being a 70's movie. I'm 27, and I saw it when I was 10 or under...maybe that would help.