Dark Day in the Deep Sea: Merlin Mission

Jack and Annie continue their quest for the secrets of happiness-secrets they need if they're going to save Merlin. But when the magic tree house leads them to a tiny deserted island in the middle of the ocean, they're not sure who needs help more-Merlin or themselves! The brother and sister team are soon rescued by a ship of explorers and scientists. But the crew isn't looking for the secrets of happiness. The crew is looking for... a sea monster!