The Raft by Stephen King
Stephen King's Skeleton Crew is one of my very favourite short story collections. It was the first single-author collection that really blew me away, and it's pretty tough picking a favourite from the book. I think The Raft is the one I'll go with here, though, just because of the simplicity of the tale. (The Mist is a novella, so it doesn't count.)
Four college students (two girls; two boys) go swimming in a lake. They set off towards a raft at the centre of the lake, and once they get there it becomes apparent that...something is in the water. See: simplicity itself.
This story scared the shit out of me when I first read it, and King's smooth, chummy prose, combined with a brilliantly conveived monster and some superbly executed scenes of tension, still manage to thrill me. There was also a pretty decent film version in Creepshow 2.