See The Depths

Finnegan Jarvis should be dead right now - drowned at the hands of the most vicious mermaid the landfolk have ever seen. He would be, if his girlfriend hadn't transformed him into a merfolk like herself. Now he's a fish, seeing the depths of the ocean, exploring the Seadom with Erie by his side. So why can't he stop thinking about the surface?

Being a superpredator isn't as magical as it initially seems. When Erie takes him hunting, the fish scream. When she tries to get physical, the alienness of his own body makes him pull away. And when the King finds out Finn's life has come at the price of Princess Clair's, Finn becomes a target not only of the King's wrath, but of nearly the entire Seadom.

Then the boats begin hunting Mer again. Finn realizes he has a choice to make: stay with Erie as a powerless Mer forever, or make the painful transformation back to human and stop them, losing her in the process.