That fishing line idea works.
I worked at a petstore when I was a kid, and we had customers with ponds and pools and your problems. One customer had a pond containing expensive koi fish, and a great blue heron would arrive every once in a while and clean him out. He then strung up fishing line around in a pattern and bird didn't like it and gave up.
But that does represent enough interference with a federally protected species under the Migratory Bird Act, so don't boast about your success to the wrong people!
Here's a company that makes a device that swings a fishing line around your yard from a tree every 30 minutes.
I've also seen water sprinkler attached to motion sensor that automatically turns on and sprays incoming birds work in certain conditions.