California Code, Vehicle Code - VEH § 9853.2

The owner shall paint on or attach to each side of the forward half of the vessel the identification number in such manner as may be prescribed by rules and regulations of the department in order that it may be clearly visible.  Any such rules and regulations shall be developed in cooperation with the Department of Boating and Waterways.  The number shall be maintained in a legible condition.  The certificate of number shall be pocket size and shall be available at all times for inspection on the vessel for which issued, whenever the vessel is in use, except as to those vessels subject to Section 9853.3.