Sounds like Alligator Snapping Turtles, although they could bite you normally they are as scared of you as you are of them. If you want to catch them and make turtle soup make a few jug lines and bait them with hot dogs. I have caught many of them this way just be carful getting them out of the water. I actually had one walking across my yard before I dug my pond heading from one neighbors pond towards another neighbors pond must have been about 35 pounds. Picked him up with a broom handle after he chomped it and relocated him to a river near the house.

Bob