Monster Ooze Through the Dock

I've been trying to find the title of this horror movie for years.

All I remember is that these college/high school kids are out at a lake and there is this "blob" like thing that is in the lake and sucks them down. It would come up through the cracks in the dock and get them.

It is not The Blob but I can't find it anywhere and no one except for me and my brother has any clue what we are talking about. Please help us.