I can't speak for other states and it may even change from county to county here, but, like RAH mentions, if a land owner's property line touches the water they have rights to the water. I would like to think that they also have responsibilities to repairs and the like. Let the dam on your property need money sunk into it and you'll find out how much your neighbor likes the pond.
The best advise I can give, beyond "avoid sharing a pond", is to try and establish a written agreement regarding usage, number of guests, adding docks/boats/waterslides/fountains/fish/etc./etc., liability insurance, sharing repairs, culling regulations, mowing and similar maintenance items, penalties for failing to comply, and so on and so on. I'm sure the list is endless and a good lawyer would be most appropriate to employ. A lawyer the specializes in homeowners bylaws would be who I would look for.
