Spoilers:
I really loved this melancholy handling of material that I could imagine being really over emphatically handled in less sensitive hands that may have emphasised different aspects of the characters and their culpability in events to a greater or lesser extent (though I do feel as if it very slightly makes the character of Isabel pay a bit too much for her pivotal choice). It feels like a paen to the lost art of letter writing, where people reveal their innermost secrets that can both build up and break down their lives as they know them. The sense of taking responsibility for one's feelings comes through extremely strongly, both positively in the beautifully sketched in pen pal romance-to-marriage of the main couple as Tom, after his experiences on the Westerrn Front and desire to take on the isolated lighthouse keeper job, begins to want to have relationships with others again, or at least with Isabel. But also negatively in the sense that once the 'baby swap' is done and nobody would ever be any the wiser Tom is placed into a situation of being unable to live with that act weighing on his conscience, even before he directly finds out the effects on the mother of the dead man and baby who wash up on their island (in the same storm that Isabel lost her baby in), with that fundamental wrong overwhelming even his love for Isabel and his new daughter that has replaced their tragically lost children. But the letter writing also returns positively again in the Tom and Isabel reunion (after Isabel pointedly refuses to read Tom's letter at first, which prolongs the pain unnecessarily), and then Isabel's final devastating letter across time, in which emotions can be expressed that might not be read until days, weeks, even decades later, but still retain their power to reframe the reader's relationship with the writer.
The main theme of 'baby snatching' perhaps bears some relationship to that unjustly neglected film
Waterland (similarly played out through the eyes of the man in the relationship who has to 'deal' with his wife's irrational behaviour in the wake of childlessness, although Waterland has a radically different structure of teacher-pupil present-past flashbacks to continually reframe its main relationship compared to Light Between Oceans' straightforward telling emphasising the natural world more) but I think that The Light Between Oceans handles it extremely well, especially in the complicated way that the 'crime' unfolds as Tom (who was really forced into the situation by Isabel, though complicit. But then he did kind of cause the death of their first child through neglecting Isabel during the storm for his lighthouse duties, which perhaps caused her subsequent health problems) takes full responsibility, whilst Isabel is wrestling with her own feelings about Tom's betrayal. But he's taking
too much responsibility (to the extent of going to prison for 'his' actions). Really the last section of the film is a kind of emotional thriller about Isabel having to 'get over herself' and the loss of the child she brought up in order to have a personal moral awakening as to the shared responsibility of what they have both done, not just Tom.
I also particularly love that the film is generous enough to bring in the emotions of the Rachel Weisz character as the actual mother, who has her own complicated reaction to the situation, going from anger to eventual forgiveness for the couple's actions, especially in that beautiful scene of remembering the dead father, which really underlines the need for the daughter to have been returned to her, no matter how good as parents Tom and Isabel were. And really it is better for Tom and Isabel too, to have had that situation resolved rather than have lived a lie which would only have gotten more complicated as their surrogate daughter grew up. It kind of turns into a film about fundamentally decent people making bad choices and beating themselves up far too much for their actions. Because at the very least they saved Lucy from the boat, which would have been better than the child dying there, even if it would have been less complicated for everyone involved, which is something that I think makes that eventual return of the grown up Lucy at the end of the film extremely important as a moment of pure forgiveness and showing the lives that would not have existed had Tom and Isabel not been there for them at all.
And in the end there is that extremely moving part forced, part chosen return to isolation in expansive Australian landscapes, but without the loneliness that implies but instead more of a sense of yearned for peace to match the similar choice of the lighthouse at the beginning of the film. At least for Tom, if not Isabel.