I would not harm a land terrapin. Someplaces they are eaten as a delicacy(Maryland). I have never eaten one and cant say.
Aquatic turtles come in may species. The only ones I have ever eaten are the so called 'snapping turtles'. Up to a foot or so long they make a good meal. I have seen them three feet from tail to snout. One this size weights 40 or 50 pounds. I can't imagine how old one this size must be, but I have never caught/killed one this size.
As a rule, you catch aquatic turtles in nets or by hand(I'm told, I ain't grabbing a pissed off snapper by hand, they can bite paddles in two) I can't see how you could harvest one for meat if you shot him. They sink and are eaten by other turtles.
A couple of years ago, I was driving down I 95 about 20 miles north of Richmond. A snapper about 2 feet high and 4 feet long had crawled out of a roadside borrow pit and was about to get in the roadway. A state police officer was off on the side trying to keep the snapper from getting on the road. I pulled over and got a 4 foot piece of 2X4 out of my van. As I approached, the trooper jabbed at the snapper with a side handle baton. The turtle grabbed it and would not let go. I told him "son, the old folks say that a snapping turtle won't let go until it thunders."
He said, "give me the board, get in your van and leave, I aim to get my baton back."
I came back the next day. My severely dented 2X4 was leaning up against the railing. There was no turtle, dead or alive in sight. He might have weighed 50 pounds and would have made a lot of turtle soup.