What's the name of this movie?

Its an old movie, probably from the seventies or eighties. I don't remember any of the actors or the name of the film or the whole entire plot. I know there was this guy, he may have been a writer. And he had this little boy who ended up drowning in the pool or the by the dock while he wasn't paying attention. And there's all these trippy monsters or ghosts or something all over the place and its a really surreal, confusing, odd kind of movie. I remember something with mirror, like one of those medicine cabinets, and he opens it up and goes inside of it at one point. Its an old movie. Does anyone know what I'm talking about?