You're his prisoner. Or is it the other way around?

You're a siren, but you can take human form. He has no idea you can do that, and he doesn't understand who steals fish from the hold and adjusts his blanket.

You have a good relationship. He doesn't hurt you, takes care of you, gives you gifts. You are his prisoner, but you can escape at any time. But you don't want to, because being a lonely siren sucks.

You tried to drown him once, but you failed miserably because he's technically dead. Yeah, he's dead, just like you.

Now you're sitting in a giant fishbowl while he's showering you with gifts.

He might mention his life as a slave, so be careful.