I talked to some people at Monona Town Hall about this parcel of land and they confirmed it is NOT private property... it is NOT his land. If he threatens to call the police, let him -- if he causes other problems beyond just yelling, call the police.
That said, it sounds like it is not officially part of the park either -- They said it was owned by the state.
I solved the puzzle earlier but didn't want to go wading in water for this one. I'll probably look for it this weekend.