I think the ghost was a very unreliable narrator. She didn't care anything about right or wrong, only her own feelings. As a pirate, I wouldn't be surprised if her Captain had done terrible things to earn her end.
I liked the underwater creatures and interactions between them. Beautiful and mysterious. The mermaids could be thought of as dealers of mercy for the doomed sailors. Is dying under a spell that brings you ecstasy and a quick death really worse than dying from exposure and dehydration on the ruined ship?