This is book number 2 in the Emily Windsnap series.
All I wanted was to impress my best friend, Shona, and the other mermaids who live in the sea around Allpoints Island. But what did I do instead? I woke up the kraken, a terrifying octopus-like monster the size of an apartment building. Now he's coming after everything and everyone I love. And who can stop him? You guessed it. Me. Well, me and Shona, and another person too–someone I'd hoped never, ever to see again.