The Sea of Terror (Once Upon a Tim #3) by Stuart Gibbs

Join Tim and his friends on a treacherous journey across the seas.

After helping Princess Grace foil the villainous Prince Ruprecht—twice!—junior knights Tim, Belinda, and Ferkle are on a new quest—this time to recover the Queen of Merryland’s precious golden fleece (and more importantly, the protective amulet in its pocket) from the Kingdom of Dinkum. The safety of all Merryland depends upon them.

But first, they must face the deadly Sea of Terror and the dastardly perils it contains. Tim and his friends will have to use every ounce of bravery, valor, and intelligence they possess to navigate the treacherous waters—or else risk becoming flotsam on the waves.