The Marvellous Submarine (the Marvellous Submarine, #1) - Conor Mccammon

In this very funny and irresistibly silly junior fiction book, The Bad Guys meet The Treehouse series, but in a brilliantly invented submarine.

Readers, be warned!
This book is set on Monkey Island and contains a super-smart gorilla called Harold, two regular kids called Poppy and Billy, and the world's greatest inventor, who happens to be their mum.
BUT it also features a villainous kidnapping, a secret volcano, pirates on yachts, a hungry blue whale, exploding basketballs – and a whole lot of water.
Plus, there's Mum's greatest invention yet: a Marvellous Submarine.