Mo. Rev. Stat. § 306.080: Vessels exempt from numbering requirements.

A vessel shall not be required to be numbered under this chapter if it is:

(1) Already covered by a number in full force and effect which has been awarded to it pursuant to federal law or a federally approved numbering system of another state; except that the boat shall not have been within this state for a period in excess of sixty consecutive days;

(2) A vessel from a country other than the United States using the waters of this state for not more than sixty consecutive days;

(3) A vessel whose owner is the United States;

(4) A ship's lifeboat which is used solely for lifesaving purposes;

(5) A vessel documented by the United States Coast Guard or any federal successor thereto; or

(6) A motorboat designed and intended solely for racing while competing in any race previously approved by the water patrol division in accordance with section 306.130. This exemption shall include operation within a twenty-four-hour period prior to such approved race in order to test or tune up the motorboat.