I had a raccoon problem several years ago when we were between dogs, raccoons usually won't come into an area where they smell dogs. I put up an electric wire like they use for cattle. Ran the wire about 6 inches above the ground all around the pond about 2 feet from the ponds edges and then ran it up and back around the pond again about 12 inches above the ground. I heard the raccoons scream the first night and they never came back. This would work for otters too but you might want to lower the wires about 3 inches. There are two types of electric fence chargers, one that sends a pulsing current and one that sends a constant current. You want the type that sends a constant current. Most animals will only need to be whacked once or twice at the most and they'll never come back. When I first put mine up I wasn't sure it was working properly. Later that evening after some beer courage I touched it, and yes, it was definitely working, about knocked the piss out of me!