Honestly, I'd probably say that it is 90% sure a heron as their beaks are just deadly. There are many different types of herons beyond just the Great Blue Heron, so maybe you've seen a different type near you (I've seen a couple different ones attack my pond). Anyway, I'd go with some type of netting at least temporarily for now. It's the only sure fire way to prevent heron/predator attacks for the most part. You may be able to remove the netting later on if you think that it damages the aesthetics of the pond/s, but at least while you're currently having a predator issue, it would be a good idea to put some kind of netting up. The predators learn and memorize where they can get their "free meals", so they'll just keep coming back if you don't do anything.