It was about a girl whose father was a professor of sorts dealing with oceanography; had house built out over water; starts she meets boy on shore, they find turtle eggs; then carve their initials on turtle's back; father is experimenting? House starts shaking one night, something in water?; girl is older, rescues guy by taking him to underwater cave (she lives there?); flashbacks to 1700s? With ship being tossed in storm; captain may promise devil his daughter for safety?; scene with old black blind woman telling guy the legend?; scene with fishing boat snagging the turtle with carved back; last scene is guy leaving area on ferry and he drops a necklace (from girl?) into the water. Obviously, can't remember movie very well, was very young, and obviously very impressionable. Neat movie though. Kind of scary/freaky.

The Bermuda Depths (http://en.wikipedia.org/wiki/The_Bermuda_Depths)