Our Wives Under the Sea by Julia Armfield

Miri thinks she has got her wife back, when Leah finally returns after a deep-sea mission that ended in catastrophe. It soon becomes clear, though, that Leah is not the same. Whatever happened in that vessel, whatever it was they were supposed to be studying before they were stranded on the ocean floor, Leah has brought part of it back with her, onto dry land and into their home.

Moving through something that only resembles normal life, Miri comes to realize that the life that they had before might be gone. Though Leah is still there, Miri can feel the woman she loves slipping from her grasp.