Sea-Serpent Island

When Adam Macadam was thrown up, a castaway, upon a Pacific Island, he at first believed that he had the whole place to himself. But first he became aware of the presence of the Lurkers - invisible beings who did not seem to wish him harm, but did seem to wish to remain invisible. And then he became aware - very much aware - of other things who did not bother to remain invisible and clearly wished him all the harm in the world