How does he have an answer to every locked door?
Answers
Answer:
person A sends key only in a unlocked box to person B.
2) person B sends key only in a unlocked box to person A.
3) 3rd trip person A puts diamonds in the box snaps the lock their holding closed. (At this point owner of the lock can't open the lock. Ensure all contents are in the box before engaging lock.)
4) once the lock box arrives to person B, they can use the key provided on the trip before to open and stare at the contents, And realizes he can pay the ferryman to give him/her ride off the island. Then place a thank you note in the box and close with lock initially held by person B and which now person A has the key.
Option B for #4 step) send a thank you note and lock with which the diamonds were so cleverly send to you sealed with. ;)
*** Now follow up question would be, how to ensure the ferryman didn't copy the keys during the first transfer of keys?
Answer) well, I guess you could have exchanged open locks and not the keys between the two people, and if he could copy a lock on a boat - the ferryman should be looking into more rewarding currier.
But why do they care about diamonds when their stuck on an island, not stranded just stuck - pay the ferryman. There is always a man in the middle, know what to trust and what not to. Focusing on security when you may have bigger problems is useless.