«The Secret of the Dark Waterfall» by Alexander McCall Smith

The School Ship Tobermory and its intrepid young crew are back in Hebridean waters. When a violent storm blows them off course to a remote island, they discover a fisherman’s journal written a hundred years ago which tells of a mysterious shipwreck crammed with treasure.
But without a precise location, where do they start looking? An extraordinary chance encounter gives them a valuable clue. Before long, Ben, Fee and their friends realize they are not the only ones searching for the lost wreck and must face a ruthless and determined adversary who will stop at nothing to seize the prize.