"Regardless of your granting wishes or no, I've still a need to avoid death. Savvy? You may not be a thing what grants such hefty boons, but there are other ways to life eternal. Help me t'find and negotiate with such beasties and I'll keep you on crew and safe til the job's done. Then I'll take you home. Seems far more fair than you should hope for, truly."
Jack sits back on the bed, relaxing on his elbows. He's not worried about the selkie running anymore. It obviously knows the danger there and believes him. Now all he needs is for the animal to agree to help him, to keep the mermaids from killing him, and to aid him in finding some means to life eternal. So, nothing too difficult, really.