Hers what CA DMV says:
What Are the Regulations for the Vessel Registration Number?
The vessel registration number (CF number) issued by the department must be displayed as follows:
Painted on or permanently attached to each side of the forward half of the vessel.
Be in plain vertical block characters of not less than three inches in height.
Be arranged so that it it reads from left to right.
Contrast with the color of the background so that it is distinctly visible and legible.
Have spaces or hyphens that are equal to the width of a letter other than "I" or a number other than "1" between the prefix and the number, and the number and the suffix.
The department issues a Certificate of Number (registration certificate) and registration stickers to identify a vessel as currently registered. These stickers must be securely affixed to each side of the vessel three inches aft of and directly in line with the registration numbers and must be clearly visible at all times. Do not place any numbers, letters, or devices in the proximity of the registration sticker.