Quick answer: The most effective legal way to get rid of geese in a pond or pool is active hazing: motion-activated sprinklers, noise, and movement, combined with visual deterrents like reflective pinwheels, decoys, and grape-scented repellent. Geese are federally protected under the Migratory Bird Treaty Act, so scaring them off is legal, but touching eggs or nests is not without a USDA/APHIS registration.
How bad can it actually get?
One year, more than 50 Canada geese decided our small backyard pond belonged to them. It got bad fast: our dogs kept eating goose droppings around the water's edge, picked up worms from it, and we ended up spending several thousand dollars at the vet fixing that. At that point it stopped being an annoyance and became an actual problem worth solving. So we solved it. This is what worked, what's legal, and what to watch out for.