"Yes. Died. Eaten. Taken to Jones' locker. Luckily, the world still had need of me. However, I've some concern that I may not always be able to count on my necessity and usefulness t'motivate others into my resurrection. Best t'assure I not die in the first place from here on."
The explanation is quick and vague. Jack doesn't have time to tell stories here, he's already been waiting weeks at sea for his reward. He's been a good guardian and well behaved. He's only periodically questioned the selkie about their appearances and generally only when crewmen had recently questioned him. He neither harassed nor made ill use of the creature, the least it could do is be quick about carrying out it's end of their accord.
"Now, let's get on with it, shall we? You've your skin and, just as soon as my wish 'as been granted, I'll see you safely set to water. Go on... Go all furry and flippery and wish granting."
A better man might wish for the lives of that tribe to be returned. Jack thought of it, certainly, but their deaths only strengthened his fear of his own.