Section 2. Every motorboat on the waters of the commonwealth shall be numbered in accordance with this chapter, except:—

(1) A motorboat owned in a country other than the United States temporarily using the waters of the commonwealth.

(2) A motorboat owned by an agency of the United States government or by a state, county, city or town.

(3) Ship's lifeboats.

(4) A motorboat numbered in accordance with a numbering system established by the secretary; provided, that the certificate of number or similar document awarding a number to such motorboat is in full force and effect; and provided, further, that such motorboat shall not have been within the commonwealth for a period of time in excess of sixty consecutive days.

(5) A motorboat numbered in accordance with the numbering system of another state, which system has been approved by the secretary; provided, that the certificate of number or similar document awarding a number to such motorboat is in full force and effect; and provided, further, that such motorboat shall not have been within the commonwealth for a period of time in excess of sixty consecutive days.

(6) A vessel which has a valid marine document issued by the Bureau of Customs of the United States government or any federal agency successor thereto.

Subject to the above exceptions, no person shall maintain, operate or suffer or permit the operation of any motorboat on the waters of the commonwealth unless such motorboat is numbered in accordance with this chapter. In any prosecution under this section wherein one or more of the above exceptions is alleged in defense, the burden of proof thereof shall be on the defendant.