"The Lake" by Natasha Preston

Years ago, Esme and Kayla were campers at Camp Pine Lake. Now they’re back as counselors-in-training. Esme loves the little girls in her cabin and thinks it’s funny how scared they are of everything—the woods, the bugs, the boys...even swimming in the lake. It reminds her of how she and Kayla used to be. Before...

Because Esme and Kayla did something terrible when they were campers. Something they’ve kept a secret all these years. They vow that this summer will be awesome. Two months of sun, s’mores, and flirting with the cute boy counselors. But then they get a message:
THE LAKE NEVER FORGETS.

The secret they’ve kept buried for so many years is about to resurface.