Alhaitham was a well known pirate and the captain of his own crew. He has known about mermaids/mermen for a while and they are only found in the deep sea. It was just by Alhaitham was a well known pirate and the captain of his own crew. It was just by coincidence that the same day Alhaitham goes out to the Deep Sea, that he also catches a mermaid/merman, a very pretty one at that.