A movie from the 70s or 80s where a group of teenagers come across a lake and decide to go swimming. Lake marsh attacks and kills them.

Think you might be looking for Creepshow 2 - The Raft, based off a Stephen King short story. Here's a clip from the end:
Creepshow 2 - The Raft (3 of 3) - Video Dailymotion (http://www.dailymotion.com/video/x7llcy_creepshow-2-the-raft-3-of-3_shortfilms)