Salty, you don't have any grasp of the issues. Which are discussed in great length a great many times in a great many places on the web and even in this forum.
What you bought is a California state registered motor
vehicle. Makes no difference that it is a boat. The boat has no national flag or national title. Whether Mexico
will recognize the state papers, from what I've been told is a matter of who you meet at the border. Since Mexico
sees a lot of traffic from California, they tend to accept it. I have no idea what the legalities are.
If you don't actually reside at the residence address you provided...most states require you to update them with any change within ten days or a similar short period. Around the same time, your insurance becomes void as well, since the address you provided is no longer yours.
Mexican insurance? I don't know.
Tax, I can tell you about. The sale
took place in California, while you were showing a California residential address, so the sales tax became due and payable to and in California. If the sale had taken place outside California waters, or if the boat had been exported within a set time frame, you might have been able to get that exempted as well. Rules vary in each state, I don't know the current
ones in California.
If you just gave the broker your California residence and didn't fill him in on all the details, he'd have no reason to discuss any of this with you. If you told the insurer "Well, I don't really live there, I live in the UK" you would definitely be treated differently. As a UK resident with a boat in Mexico, no matter how it got there, you'd probably need your cruising area extended from "nearshore" to "global" and thepremiums would go up to match.
Mexico? Deserved or undeserved, has a reputation in the US for being sometimes very casual about very many things. Although if you have a car and the insurance on it is't completely in order, a slight accident
can result in longer discussions from the jail cell. I hae no idea how they'd treat invalid marine
insurance, but I'd suggest you start asking questions from the people who have to enforce these things, and tell them the longer story.
Or, just keep flying under the radar
and be very careful about how you do it.