Loved this film as a kid, it was early 80s, vague memory but I think it was about 3 children at some sort of lagoon, but certain it isn't the blue lagoon or creature from the black lagoon. Pretty sure I felt nervous when they jumped in the water so there may have been a menace of some sort but it wasn't terrifying. Watched it loMoreno frustrated no more details at present.

Any chance they were teenagers instead of children? I wonder if you're looking for a segment from Creepshow 2 called 'The raft'...