If I were brave enough to do this without consulting a lawyer, I would sit down with the owner, and write out exactly what was expected of me: taking the boat to a port, making all decisions relative to the welfare of the boat and crew while it was in your custodianship, define what stops your custodianship (the owner's arrival, whatever). Make sure you have copies of all papers relative to the ownership
of the vessel, 'cause you may wind
up acting as a proxy for the owner.
Even better, find someone else who has done what you propose, recently, to learn what would be required.
Or get on the phone
or web to the Mexican Consulate and get it from them, preferably in writing, so that you can follow their instructions.
I think the last option is the best. ^^^^