A couple of things have not been mentioned:
will want state sales taxes
(Around 9 %) depending on the county you buying
the boat in. There are ways around that... Take an off-shore delivery
and remove the boat from the state within 90 days. A Marine
Title person can assist you with this...
You can de-register the boat with the USCG, but obtaining a California
State Boat Registration will but you back in the tax circle, so you definitly want to consider documenting the boat in another country or registering it in a US State with favorable tax laws.
If you are taking a loan to purchase
the vessel, the loan company will most likely demand the vessel be documented, especially since you will be traveling internationally. They will want a means to take possession of the boat if you fail to pay your mortgage.
Finally, as suggested previously, if you have troubles getting into the USA to take possession of your vessel you can arrange the title transfer through a Marine
Title Office and then have the boat delivered to Mexico
. You didn't say where it ws located, but assuming it is in South California a delivery skipper
would charge less than $700 to take it to Ensenada, Mexico