Okie dokie so here's the tea.

Person A is a pirate who's known for killing/selling sirens (not particularly mermen but he doesn't really care if there's a difference lmao he doesn't know that Mermen are the ones who can also turn into humans so he's like n a n I) because his mother was killed by one when he was a young boy. He ran away from his cold and verbally abusive Padre (daddy issues :') )) to go start his own life out on the sea with his friends who become his crew, while simultaneously getting some revenge

Person B is an innocent, curious lil merman who saw the ship and went Yolo, got trapped in its net, and the rest is history my dudes.

Eventually person A and B, after lots of angst, slow burn, and a bit of mutual pining, fall in love. Oh and if you want Person B could have a friend that either tagged along or cane to rescue them and could have some romance with the second in command ;))))))