I've read about people using a motion-sesor water jet system. They aren't cheap, but they supposedly make a good non-leathal deterrent. Though I wonder if the animals it's supposed to chase away just get used to it over time. And then of course, you have to remember to shut off the motion sensor before you go out to the pond or you'll get sprayed, too.
I'm not sure how fish-safe the stuff would be, but you could use cat-deterrent spray. They sell it for gardens, so it's plant safe, so maybe it would be safe enough to use around ponds.