Buying a US
boat in
Mexico can be either easy or difficult, likely both!
You will owe no
taxes until you return the boat to the US. Then the
taxes (usually "use" tax) depend on the state and locality where you site the boat. There are no Federal taxes involved, just the slight cost of documenting the boat with the coasties (assuming it is a documented vessel).
Regarding buying the boat.
Purchasing a US boat in Mexico is actually illegal, so legally the boat needs to change hands while in international waters, just like taking an
offshore delivery in the US. Few purchasers go this
route and most brokers have work-arounds. One sticking point may be the TIP. If the
current owner has a valid (and unexpired) TIP, it must be cancelled and a new TIP (in your name) obtained. Again, a good
broker may be able to facilitate this. If the
current owner is unavailable or the current TIP is expired, the process can be long and involved, with unbelievable bureaucracies to contend with. If buying directly from the owner, you might be well advised to use a local (in Mexico)
documentation agent.
Also, depending where in Mexico the boat is, it can be easier than other locations. For example, a boat in Ensenada where it is easy to go "offshore" and all the necessary offices are co-located, it can be quite easy.
San Carlos is a long way from "offshore" so more work-arounds may come into play.