A boat fee is hereby imposed on all boats, except boats in the following exempt categories:
I. All boats under 10 feet in length, including ski craft as defined in RSA 270:73, V, under 10 feet in length.
II. Canoes, kayaks, rowboats, sailboats under 20 feet in length, sailboards and sailing canoes.
III. Commercial boats.
IV. Boats that are exempt pursuant to RSA 270-E:4.