There is a forum post on using a floating PVC square turtle trap as a pretty easy way to get turtles. I'm not sure if snappers no how to elude these or not.
The only time I have been able to get a snapper out is to catch them with hook and line and then tow them to shore and dispatch of them with a pellet gun to the noggin while someone holds their head out of the shell with the fishing line. You could probably set out a 'jug line' or shore line baited with anything meaty or smelly and catch one pretty easily that way.
