On thin ice / Sibéal Pounder ; illustrated by Jason Cockcroft.

In the kingdom of Frostopia, mermaids Beattie, Mimi and Zelda have come to the aid of Maritza Mist, a famous water witch who supplies magic spells to the underwater kingdoms. Two notoriously bad mermaids have escaped from Viper View prison and stolen Maritza's map that shows the locations of twelve magical objects. When brought together, these objects will create the most powerful water witch the world has ever known. Beattie, Mimi and Zelda set off to travel the underwater kingdoms to stop them. But they haven't counted on meeting a bunch of over-zealous spies, the oldest shark in the world or their faithful companion Steve the seahorse going rogue...