The entire time that Ginko takes to think, the merman doesn't take his eyes off of him, doesn't stop smiling. It's debatable if he even blinks. Instead, he almost stays as still as anything, and whatever breath he takes can't be heard over the sound of the waves hitting the boat.

Mermen speaking Japanese is a mystery that will have to be for another time, but mermen do seem to understand nonverbal peace treaties, or at least the act of putting the sharp object down. In a heartbeat, the smile vanishes from the merman's face, leaving something otherwise blank but no less intelligent.

It seems that he's waiting for something. Most likely Ginko's next move.