fishrgurl - It does not matter if someone can get to it without crossing over you property. If you own all of the pond, every little bit, then you can decide who fishes those waters. Think of it like a fence in your backyard. Your neighbor can legally approach your proprty to a certain point, but may not legally cross that line without your permission. If even a miniscule amount of that pond is under that fence, your neighbor would legally have the right to go anywhere on that body of water. This might occur if the pond expanded due to heavy rain or something like that.

Now having said that, if your neighbor was on the pond legally and got out of his boat, or maybe he is swimming, the very second he touches bottom, he is trespassing. If the water is communally owned he has access to it, but the land UNDER the water is yours and you can decide who can walk on it.