Manjiro was the captain of the pirate ship, him and his crew were travelling overseas to find treasure

The crew was havin a feast inside the ship, while Manjiro was out at the deck of the ship looking out to the horizons, wondering if he will find the treasureso he can heal his sister, Emma, who was seriously ill

As he thought about the treasure and healing his sister, something caught his attention, a pair of(eye color)glowing eyes looking up at him from the water, it was mermaid/merman