The Secret Tide

When Tom and Molly Shuttle were forced to stay with their strange Aunt and Uncle Hobble and even stranger cousin Seymour they thought their summer holiday was over before it had begun.
But all that was about to change when they met the locals of Willow-by-the-Sea.
'There's only one thing to remember while you're here,' Seymour had told his cousins, 'Keep away from the old house on the rocks. That's where the Dark Man lives.'
Soon enough though Tom and Molly's adventures lead them to discover that all is not as it seems with the Dark Man. He has a secret buried not only deep in his past but also deep beneath the rocks where the tide ends and the cliffs begin.
No one else has ever seen the secret tide until now.