A small wooden post that works like a hitching post - should also require a rope to make it a little expensive/difficult to spam on the field. Stealing the boat leaves a scent even without a claim (pretty much like a Leanto works)
Edit:
More simple idea:
no scents or anything, just allow us to hitch a boat on a tree. To take the boat you need to chop down the tree. Annoying enough for most people to not just take boats away for the fun of it
