See It! The Long Weekend (1978)

This has already been remade and is due for release on DVD IO believe, it was supposed to go into theaters but has been sitting on the shelves for a little while now. Check Netflix for the remake that stars Jim Caviezel.
This 1979 version details are as follows: When a suburban couple go camping for the weekend at a remote beach, they discover that nature isn’t in an accommodating mood.