Wild Dark Shore by Charlotte McConaghy

Dominic Salt and his three children are caretakers of Shearwater, a tiny island not far from Antarctica. Home to the world's largest seed bank, Shearwater was once full of researchers. But with sea levels rising, the Salts are now its final inhabitants, packing up the seeds before they are transported to safer ground. Despite the wild beauty, isolation has taken its toll on the Salts. Raff, eighteen and suffering his first heartbreak, can only find relief at his punching bag; Fen, seventeen, has started spending her nights on the beach among the seals; nine-year-old Orly, obsessed with botany, fears the loss of his beloved natural world; and Dominic can't stop turning back toward the past, and the loss that drove the family to Shearwater in the first place. Then, during the worst storm the island has ever seen, a woman washes up on shore.