Technically you need to have CF numbers on any motorized vessel...
If you visit Redondo's King Harbor and motor or fish around the harbor you will eventually be stopped and checked by the Harbor Patrol if they notice you have a motorized vessel without CF Numbers...
If you just motor directly from the ramp out into the open ocean they MAY NOT notice you.
I have been stopped and checked for a license and PFD while trolling in the harbor and the the Patrol officers commented that having numbers was the proper way to operate.
It's really pretty cheap and easy to do at AAA as Raskal commented... and it's good insurance on being able to stay on the water and not ruin a day just because a Harbor or Lake Patrol guy decided to stick to the rules...
