Next Up: Raider's Ransom by Emily Diamand

(From inside cover): It's the 23rd century and much of England --what once was England --is underwater. Poor Lilly is out fishing with her trusty first mate, Cat, when greedy raiders pilliage the town --and kidnap the Prime Minster's daughter. Her village is blamed. Lilly decides to find the girl.
Off she sails in secret. And with a ransom: a mysterious talking jewel.
If she saves the Prime Minster's daughter, she might just stop a war
Little does Lilly know that it will take more than grit to outwit the tricky, trecherous pirate tribes!