The pirate didn't particularly mean to threaten the mermaids. He just needed a moment of quiet panic to himself. Still, what's done is done. He also didn't know he was making a trade with one of them. He sees that two of them are angry and that simply won't do. Clearly, they don't understand english. This will pose a bit of a problem.
He sees the blond go for his hat and snatches it away, putting it firmly on his head.
"No. My hat. Not for you. ... Look.." Jack searches the inside of the boat and pouts as he finds his rum to be the only other thing of real value that he can part with. He holds the bottle out to her and uncorks it. He takes a drink to demonstrate its use, then offers it up. "Here. Rum. It's good. Better than pirate."