Dolphin's grace / Maggie Marks.

Enhanced descriptions from Syndetics:

Follow along with brothers Mason and Asher in the third Aquatic Adventures in the Overworld book!

Brothers Mason and Asher have only ever known the comfort of dry land in the Overworld. But when a terrifying shipwreck leaves them stranded, their new friend Luna urges them to safe haven-- underwater. In each story of the all-new series for Minecrafters, Aquatic Adventures in the Overworld , this unlikely trio must make the choice of staying in the world they know best and risk death, or voyaging into an unknown world where anything could happen. Follow their journey as Mason, Asher, and Luna face incredible obstacles and discover strength they never knew they had in Aquatic Adventures in the Overworld.

When Asher doesn't return from his hunt for buried treasure, Mason wonders what became of his brother. Then Luna appears, insisting that the pod of dolphins swimming nearby have a message for Mason. Do they know where Asher is? To find out more, Mason and Luna have to first earn the dolphins' trust. But when danger strikes, Mason wonders--is it too late to save his brother, and can he and Luna save the dolphins, too?