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 may have come back wrong. Whatever happened in that vessel, whatever it was they were supposed to be studying before they were stranded on the ocean floor, Leah has carried part of it with her, onto dry land and into their home.