Marine Madness — Megan Miller

The sixth and final installment of New York Times bestselling author Megan Miller's S.Q.U.I.D. Squad series.

The S.Q.U.I.D. Squad is made up of three bold children who are part of a secret, underwater society of Book Guardians. They combine their unique talents to solve mysteries, right wrongs, and explore the oceans!
 
While settling into their new base at Ocean's End, life returns to business as usual: books are being delivered and stored and everyone is at work. The only new trouble in their lives is a pair of twins who really don't want to be there. Troublesome turns to puzzling when the adults begin to behave oddly. First, Sofi can't stop laughing. Abs begins making silly jokes and playing with toys. To Inky and the gang's surprise, Ocean's End headquarters slowly transforms into fun and games. But when the S,Q.U.I.D. Squad realizes that the adults are suffering from a magic that turns them into kids, the joke is over.

As the squad works to find a cure for their parents, another troublesome change occurs. The fish and sea creatures that used to be friendly begin attacking Ocean's End, and the magic barrier keeping them hidden starts to break down. If the SQUID Squad can't find the source of the mad magic and stop it, their noble mission will be exposed to the Pillagers and their lives will be in serious danger.