The Light Between Oceans by M. L. Stedman

Tom Sherbourne, released from the horrors of the First World War, is now a lighthouse keeper, cocooned on a remote island with his young wife Izzy, who is content in everything but her failure to have a child.
One April morning, a boat washes ashore carrying a dead man - and a crying baby. Safe from the real world, Tom and Izzy break the rules and follow their hearts.
It is a decision with devastating consequences.