I would avoid "a shared pond" if at all possible. An earlier shared pond is the sole reason I own my current pond.

Of course you may have a wonderful "one of a kind property" that you are in love with and just have to have it.

So not sure what your situation is, but if it's still possible I would have an attorney draw up a 25 year agreement that states the rules and includes you getting first right to purchase or consider forming an HOA that you control. If you do an HOA any future owner of the other property would also be under the HOA rules. I'm not a lawyer, but I would see if forming an HOA is possible.

Fishing has never been about the fish....