Escape From Atlantis by Kate O’Hearn

This is amazing! The same author also wrote the Valkyrie series and the Pegasus series, both of which are super great. Riley and her cousin Alfie hate each other. To make matters worse, they’re stuck on a boat with Alfie’s mom and Riley’s dad. But then their boat is capsized by a leviathan! Riley and Alfie are separated from their parents and stranded on an island called Atlantis. The people there are…odd, to say the least. There’s no technology, and no one seems to care that Riley and Alfie’s parents are missing. But after a few searches, they find out their parents are dead. Riley can’t believe it. And that’s not all. As people on Atlantis get old, they start turning into animals and are then shunned. Riley thinks this is awful. Riley and Alfie search for clues about their parents’ demise and also try to find a way off the island. Will they escape, or will they be stuck on Atlantis forever?