Simon Thorn and the shark's cave / Aimée Carter.

Twelve-year-old Simon Thorn's life is almost unrecognisable from just a few months ago. He has a twin brother, an uncle he never knew and his first real friends at the secret Animalgam Academy. Not only is he an Animalgam, he's also the Beast King heir along with his brother to a bloodline more powerful and dangerous than any other ...In this final instalment in the action-packed trilogy, Simon must venture into the water kingdom. But danger lurks in the shadows beneath the waves and Simon will have to call upon his friends and all he's learnt to survive ... and save the world. Kingdoms collide and the stakes are raised in this electrifying final novel about a boy who learns it's not the powers you're born with that make you a hero it's how you use them. Ideal for fans of Percy Jackson and Beast Quest.