Cal. Veh. Code § 9850
Every undocumented vessel using the waters or on the waters of this state shall be currently numbered. No person shall operate nor shall any county, city, or political subdivision give permission for the operation of any undocumented vessel on those waters unless the undocumented vessel is numbered in accordance with this chapter, or in accordance with applicable federal law, or in accordance with a federally approved numbering system of another state, and unless (1) the certificate of number issued to such undocumented vessel is in full force and effect, and (2) the identifying number set forth in the certificate of number is displayed on each side of the bow of the undocumented vessel for which the identifying number was issued.
Ca. Veh. Code § 9850