Between Bloode and Water by Marie Harte

Powerful sea witch bewitching a vampire for power. Check.
Freeing said vampire from your crazy mother? Check.
And mate—when the spell backfires and now he thinks he loves you.

Kaia has had it with her mother. Talk about touchy. Offend the sea witch and pay the price. Oy. Nearly sacrificing Kaia’s last boyfriend to a dark god had been totally unnecessary. And those deals her mother keeps making with demons to obtain more power? Not a healthy way to achieve balance.
To stop her mother from sacrificing some other poor sucker, Kaia sneaks into her mother’s lair and finds more intended sacrifices. But one of them has fangs.
Needing to get to safety before her mother returns and loses her mind, Kaia, a sea nymph, swims away with the vampire and soon learns he’s under a spell.
In order to cure him, she overrides her mother’s magic with her own, weak though it may be. But now this fanger—a freaking death-bringer who can swim like a merman—thinks he loves her. Curing Orion of the enchantment should have ended their association, but he’s still obsessed with her.
And for some reason, she’s obsessed with him as well. But the dark one to whom her mother promised a soul doesn’t care where it comes from. And since her mother has reneged on Orion, the dark one decides Kaia will do nicely in his place.