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.
"A riveting, melodramatic mystery for true-crime fans."--Kirkus Reviews

"This YA debut is a quick page turner with a lot of action to keep readers engaged... Fans of thrillers will enjoy this breezy but gritty read."--School Library Journal

"Girl Overboard kept me turning the pages. I finished it one sitting, rooting for Izzy the whole time.  I felt like I was on board the cruise ship with her, taking part in the fun and trying to escape the very real danger."--April Henry, New York Times bestselling author of Two Truths and a Lie