Being a pirate often means being out at sea for a long period of time, nothing new to Alhaitham. he feels at ease on the sea, the air is fresh and there's no one around to order him around and make him feel stressed

"Pull the nets up!"

he orders his men and watches as the nets are brought up from the sea and put on deck...this time...they didn't catch a fish...they caught..

"A mermaid-" Alhaitham says in utter disbelief as he crouches down at the mermaid before chuckling "Well, well"