It is the ship the selkie is getting off Jack in waves. But the seal doesn't believe him because a ship is a thing. People don't long the same way for things as people. Except for Jack. But it could be the pirate's lost girl Anaba. Who knows?
The seal back's up, expecting Jack to hit him. If the pirate is ever going to start it will be now. But Jack doesn't. He looks angry as all get out but he doesn't hit. As the pirate speaks, the selkie blinks at him, confused. It takes a long moment to realize Jack is asking about the mermaids. "No. I... what?" Sometimes Jack is hard to understand. "The mermaids.. well.. of course I can speak to them. With my body and my mouth." It's a silly question. How else would you talk to a mermaid? "Most things in the water don't speak. They move. I don't understand what you want."
Jack is trying to get him to agree to something but the questions are so strange to him he's lost track of what that could be.