The Castaways by Elin Hilderbrand

One summer day the lives of a group of friends are changed forever. Married couple Greg and Tess MacAvoy drown during a sailing trip off the island of Nantucket while celebrating their wedding anniversary. In addition to leaving behind their young twins, the survivors include their best friends-- three other couples who did everything together with them. Andrea was Tess' cousin and her husband, Ed, is the Chief of Police. Addison was having an affair with Tess even though he was married to Phoebe. Delilah was the closest to Greg despite being Jeffrey's wife. Throughout the book and from each character's perspective, we see how the friendships began and the grief they feel over the mysterious deaths. Hilderbrand has a deft hand at portraying feelings and at writing about everyday island life. I had a hard time putting the book down. Definitely one of her best.