Big Kahuna, even if you have lost
your prior copies of a 'Captain's letter', surely you have a good idea of what it said. You (or preferably the owner) really should check with Mexican authorites as to what they require upon entry into their country - especially considering the issues that are currently being discussed in other threads on this site. I would put the responsibility in the owner's hands as he or she is the one who is at risk of having his/her boat impounded if the paperwork is not in order!
For your purposes, I suspect that you are probably safe so long as you can establish to their satisfaction that you have the authority to be in posession of the boat. If the Mexican authorities do not have an official form, you really should speak to a lawyer who practices in Mexico
If you are not prepared to go to that expense, then you should try to speak to someone who has taken a boat into Mexico for the owner. If you are unable to do that, you should at least get someone to translate into Spanish the letter that you have used successfully in the past.
MY spanish is a bit rusty, but I assume in past you have has something like this (and sorry, my keyboard is set to English):
Autorizacion del Propietaro de Barco
Yo, name of registered owner
, soy el propietaro registrado en country where boat is registered
del velero registered name of vessel and registration number.
Yo doy Capitan your name as it appears on your passport,
numero de pasaporte your passport number, your nationality,
autoridad total sobre el uso de este barco, a su sola discrecion, entre las fechas de date of commencement
y date of cessation of appointment
El es mi representante a todos los efectos en relacion con mi barco.
The document should be signed and notarized (with a seal) by the owner. In my experience with legal
documents in hispanic countries, they often require finger (or at least thumb) prints and should include notarized copies of your and the owner's passports.
Again, though, I really think that it would be wise to check in advance with the Mexican authorities. Good luck!