Pip Bartlett volume 3
Pip and Tomas have dealt with magical creatures that burst into flames and runaway unicorns. Now they are headed with Tomas's family to Port Candor, a seaside showplace full of magical sea creatures. Soon they are surrounded by Slimekrakens and Tubafish and Spinnerseals and other dazzlers of the deep. And they are involved in a big mystery that involves a sea monster that might not be staying in its tank.