Generally, every sail-powered vessel over eight feet in length and every motor-driven vessel (regardless of length) that is not documented by the U.S. Coast Guard which is used or on the waters of this state are subject to registration by the Department of Motor Vehicles...
The following vessels do not have to be registered in California:
* Vessels propelled solely by oars or paddles.
* Nonmotorized sailboats that are eight feet or less in length.
* Nonmotorized surfboards propelled by a sail and with a mast that the operator must hold upright.
* A shipâ€™s lifeboat (a dinghy is not a lifeboat).
* Vessels currently and lawfully numbered (registered) by another state that are principally used outside California.
* Vessels brought into California for racing purposes only (exempted only during races and tune-ups).
The last one is interesting though...