Good advice, you should be able to do the same thing with the trailer, they must have belonged to the same person. Trailer might be easier if it has a licence plate.
Bob
In California, the trailer's license is completely separate from the vessel it carries.
But as I look at the picture I can clearly see the registration number. In any event... a trip to the DMV website, or down to the office... will tell you what you need to know.. you'll have to request a new owner registration. You want the state you register the boat in to tell you what they require.
But.... wait... this is a job for the seller. If he wants to "GIVE" you the boat, then you might possibly do the leg work... but he should pay for it. Otherwise, he can't sell you something he can't prove he owns... not without paying some fees ... i.e. unpaid sales tax, license fees etc. and giving you a registration...and Ca. requires a "mussel" sticker now for all inland waters.
So, it's just like buying/selling a car from a private party. To operate the boat you must have current registration and proof of ownership. To keep it in a marina it must be insured... do your homework and stay protected.
Bottom line... research the CF number with the DMV.