I think there was a recent thread regarding cats as potential fish predators. My feeling is that your visitors are most likely looking for a water source. As to whether they are a danger to your fish..... Who knows! But I would be worried if, as you say, one got stuck under the netting. Maybe if you can't keep the cats away, at least make the pond safer for them -- or for any visitors -- and secure the netting so they can't get underneath [good luck with that, though].
We have a few neighborhood cats that visit our yard. On the one hand, they keep the rat population down, but on the other hand they upset our indoor kitties by hissing and growling at the windows! We're always chasing this one big orange tabby out of the yard. I don't think our visitors are feral, just owned by ********* people!