looking for 2 movies from around the 60's and 70's

I am looking for a few movies that I watched as a child growing up. I don't know the name of either one so maybe someone can help me. I can tell you what they were basically about. The first movie was about this big gigantic turtle that lived in the ocean and could turn over ships by going underneath them. I don't remember why he done that. The next movie I remeber the ending, there is a lady in a boat fixing to leave an island after they thought it was safe to go. A ghost comes up out of the water and I think she grabs her. Anyway the movies has supposedly ghost in the lake that are swimming around. And last but not least the other movie is about the crazy Dr that is in to snakes and he is giving this guy shots of venom and it is slowly turning him into a snake. At the end he is turned into a snake. Can anyone help?? Thanks

The gigantic turtle one could be "Gamera: Gaurdian of the universe".
"Ghost Lake" sounds about right for your second request.
I believe, but I'm not entirely sure, that the third sounds alot like "Death Curse of Tartu"

Mind you. I may be wrong, but after googling the searches these seem like the best bets.

The movie about the guy turning the guy into a snake is called "Sssss" and it was a very young Dirk Benedict before the A-Team and Battlestar Galactica. That was a TV movie when I was a kid in the 70's.

As far as the Gamera films, if you saw a B&W film, that was the first film from 1965 called Gammera The Invincible. Other than that it sounds like it could be a later offering in the Heisei series of Gamera films by Shusuke Kaneko.