The Secret of Roan Inish.

Ten-year-old Fiona is sent to live with her grandparents in a small fishing village in Donegal, Ireland. She soon learns the local legend that an ancestor of hers married a Selkie – a seal who can turn into a human. Years earlier, her baby brother was washed out to sea and never seen again, so when Fiona spies a naked little boy on the abandoned Isle of Roan Inish, she is compelled to investigate.