In Mass any pond over 10 acres is considered a "Great pond/lake" and you can only own to the waters edge if you live on it. Sounds like NH has the same law. With that being said the pond I own is 42 acres but is man made, it is a old Mill pond made over 200 years ago for powering many different mills over the years. I am able to own the pond because it is man made the original owners owned the land before it was turned into a pond (by damming up the river that ran through it) after the pond was created they still owned the land underneath and that is the title I have. The original owners also had riparian rights that went up and down the river one mile each way. You would own the land you are flooding and I would think you would still own it after the fact.