The selkie doesn't want any of this. He wants to go home. Now. But Jack is right. Going through with this deal is much better then being left here, being beaten, or being a slave on the ship. And considering he's lied to Jack it's better then he thought he would get. But to have freedom constantly put further out of his reach is maddening.
"What you seek isn't natural. You're shortening your life by trying to make it longer. I can't protect you from yourself. Or them. I'm telling you the truth, Jack! I can't even escape you. Take me to cold waters and let me go. I'm not for you to keep like this."