Waves by Sharon Dogar

Hal is use to spending summers at the beach with his family, but this year things are different. Charlie, Hal's older sister is in a coma in the hospital back home. She's been there ever since last summer, when Hal found her unconscious in the water.

The memory of finding her has haunted Hal ever since, and he needs to know what happened to Charlie and how she ended up in a coma. Was it an accident or did someone hurt Charlie.... and why is it he can hear her voice in his head!