Megan Atwood 
Revenge of the Kraken [EPUB ebook] 

Jordan isn’t thrilled about spending a seaside summer with his grizzled grandpa, Captain Joe. He barely knows the guy – and he knows even less about the ocean. But his grandpa’s nautical charm soon hooks Jordan on a summer at sea. After all, who could resist his tall tales of mystical white dolphins and sinister sea monsters? But when Captain Joe’s stories turn out to be true, will anyone escape the Kraken’s revenge?