It's good to see some kind of understanding from the selkie. Jack was afraid they would simply have to continue failing to understand each other in the slightest until the time comes to part ways. The sea is something they can agree upon, though, and it's a good common ground. Only sea creatures and old sailors have the ability to love the ocean so dearly even when they know her so well.
He's intrigued by the seal's description of how he's learned his languages and how how long he's been listening. The thing must have quite a long life. Jack is jealous and feels even more sure that the selkie can, in fact, grant his wish. He smiles happily and is glad when the seal asks to be taught more about the ship. He'll need to know these things if he's to be Jack's translator.
"May as well begin your lessons now, eh?" Jack laughs and begins walking his assistant around deck. He points out the different parts of the rigging, the distinctions between the sails, the forward and aft braces, the booms and rat lines...everything he can think of, and likely all too fast. Jack just gets excited. It's how they spend the better part of the day. Jack keeps the selkie close so he can keep the crew off of him. He doesn't know how long it will take them to 'get clean' but they'll need to for both the selkie's sake and their own. For now, they're happy to be going home.
It's evening by the time Jack has finished with his lessons and they're already heading into warmer southern currents.