Giving your fish a good hiding place is an excellent start. A few more things to try - I got these ideas off Fishgeeks.com so can't take credit, btw.
Heron decoy - they are territorial birds, so if your heron sees another bird already at the pond, he will fly on by. There are beautiful life-sized heron sculptures available (for a price of course) but I have so far "made do" with a little tin cut-out heron that stands in front of my water iris in plain sight.
Lizard or alligator decoy - I have a couple of ceramic lizards by the pond. They look nice, and I hope they help keep the big birds from coming to fish.One guy got his tail broken off, and it is at the bottom of the pond somewhere, but we pretend he's coming out from under the waterfall.
Steep sides, deep water - if your pond has a gradual slope, or shelves for plants you are really inviting the raccoons to come and fish. Luckily I've not lost any fish to raccoons, but I did find one little guy swimming in the pond one night. The sides of my pond are vertical and the water is about 8in down from the walkway around, so they really can't reach the fish. The 'island' is a sculpted piece of concrete set on cinder blocks so the fish hide under there, too. It does have one point where the cat has found she can stand, close enough to lean down and get a drink. But she has never tried to get at the fish.
Fast, smart fish - when we moved into this house (and I had never had a pond before) our pond was stocked with 6 beautiful big feathery Orandas. The slowest, most awkward swimmers I've ever seen. On top of that, the previous owner had "trained" these fish to eat floating food, so whenever anyone walked by on the way to the front door, they would come up to the surface and ask for food. When the white heron showed up a few months after we moved in, of course the fish came up and they were . . lunch. I looked at replacing the Orandas (in my initial ignorance) but they were really expensive. That heron ate about $200 worth of fish in 2 days! I've stocked the pond with cheap goldfish from Petsmart, and have never given them any floating food. They eat bugs and algae, and are healthy, fast, and in the last 4 years I've only lost one or two fish. They started out about 2in. and at least one is over 6in. long now.
