Neptunia

Cassie Georgiana Odysseos has the potential to become and Olympic swimmer. However after her parents separate, her training is interrupted when she and her little brother Timmy are sent to live with elderly Mike and Peg Calypso in Ithaca, a small country town without a training pool. 

Asked to deliver an important message to the underwater city of Neptunia, Cassie must use all her strength, strategy and spirit to survive a marathon wsim. But can she overcome King Neptune's terrifying obstacles?