Whats the scary movie where the girl’s swimming, and when she comes up for air, theres glass covering the pool?

That happens in The Legacy (1978): http://www.imdb.com/title/tt0079450/
aka The Legacy of Maggie Walsh

Summary:
“Six beautiful people arrive for a weekend in the country. Six guests of the unseen host who lies wasting away upstairs…Five heirs to a mysterious Legacy watch the body of the sixth float to the side of the pool…Four claim the unspeakable power of the Legacy, while a red stain spreads across the ceiling…Three realize that the Legacy has come to claim THEM, while outside dogs dine on what was once a man…And then there are two.”

Trailer: http://www.youtube.com/watch?v=vcZdNIW42xI

Pool Cover Plexi Video