You don't need anything to buy your ticket down, just to check in at the airport
(and sometimes to clear C&I at your destination).
Presumably you have a copy of your US Documentation
for the boat. As owner, you can write a letter on your own behalf stating that you are flying to Trinidad to join the boat with the intention of leaving Trinidad by sea. I can send you a draft
letter if you like.
A fax or scan/e-mail of the cruising permit
would dot the i's and cross the t's but shouldn't be necessary.
Think of the paperwork (letter, Documentation
, Cruising Permit) as the equivalent of an ongoing ticket. It assures the airline that they aren't going to get stuck with repatriation costs if you get in trouble, and the T&T government
that you aren't planning to become a burden on their society.
Incidentally, schedule is a moving target. I don't buy my tickets home until I have feet back on the ground. Simply too many things that can and have changed.