Girl Overboard

This fast-paced thriller about a girl who investigates her friend's disappearance during their cruise ship vacation is Ruth Ware’s The Woman in Cabin 10 for teens.

When Izzy meets Jade on a cruise to Bermuda, her new daredevil friend turns Izzy’s boring family vacation into the trip of a lifetime. Until Jade goes missing.
 
The investigators claim Jade fell overboard, but Izzy knows better. Her friend had secrets—secrets that might have gotten her killed. As Izzy digs deeper into Jade’s disappearance, she realizes that someone doesn’t want her to find the truth. And if she’s not careful, Izzy might not get off this ship alive.