The Light Between Oceans by M.L. Stedman
All plot spoilers have been omitted from the comments.
Most of the group didn’t enjoy the book, especially the story line, e.g. contrived, too predictable, especially the ending. Even those who liked it thought it was lightweight, formulaic.
It was thought the author didn’t draw out the characters very well, especially their motives.
However, the story of the lighthouse lifestyle was extremely interesting.
The moral dilemma wasn’t drawn out well enough.