I can't remember the name of a book about a guy who's sent to a remote island to be a lighthouse keeper or something similar, to take over from another guy who'd gone crazy/degenerate because of this woman from a kind of...mutant sea-people population? And the protagonist gets obsessed with her and has the same kind of mental degeneration.
This sounds like Cold Skin by Albert Sánchez Piñol.
From the synopsis on Wikipedia -
The novel chronicles the story of a former fighter for the independence of Ireland who, unmotivated by the events of the Western World, decides to escape from the society in which he lives. He accepts a job offer as a weather official on a remote island in the south Atlantic close to the Antarctic Circle.
On this island there is only one inhabitant, the signals official Batis Caffo, who does not help the Irishman, and hides all the information which he has on the island. So the hero has to spend a night alone, where he suffers the attack of strange monsters that are similar to frogs.
Through his cunning he manages to stay in the lighthouse with Cafo, and thus he can withstand attacks from the big frogs. After some days, he finds out that Caffo has sex with a domesticated monster, the female Aneris, not only domesticated but also his lover. Although the Irishman first thinks that the monsters are evil and murderous, after meeting Aneris, he changes his mind.