As with any firearm, be careful. Projectiles can and will bounce of water, so be sure to not shoot near or towards any houses or animals or anything else you wouldn't want to hit.
Hubby use to shoot turtles on the pond, it's a losing battle though unless you have a TON of time and patience. If you don't have that extra time, or just choose not to use it shooting turtle heads, simply research turtle traps and go with that route. It tends to be a bit safer with less collateral damage. Also, if you go with the turtle trap route, research turtles in your area and do some research. Not all turtles feast on waterfowl and some are even protected I believe. Best to remove the snappers and turn the non-hostiles free.
-Kim