This was an English colour movie I saw on TV in the early 1970’s. Movie scene I remember is of a young lethargic and very weak woman who wakes up in this cave and realizes her blood is slowly being sucked out of her neck (and wrists, I think) by these creatures. She and her boyfriend had been either swimming in the lake or were in a boat when they were attacked from below. I think that there were lots of disappearances of local teens and eventually the sheriff and townspeople blow up the lake and kill the creatures. Because of that movie, every time I drive by a small, dark lake or bog, I get a weird and eerie feeling!
Sounds just like 1959’s Attack of the Giant Leeches (http://www.imdb.com/title/tt0053611/), except that wasn’t in color. Maybe you saw a colorized version?