One popular summer school reading theme is the beach. If students can't be at the beach, then they can read about it and enjoy the character's interactions with the sand and the waves.
For teens, try Nicholas Sparks's book The Last Song. This book takes place during the summer and mostly on the beach, with a sub-plot of saving endangered turtle eggs even. Sparks explores the themes of divorce, independence, first love, peer pressure, musical talent, class, and chronic illness. All that rolled into one book. Some teens may have already seen the movie, but the book is still worth the read.
For middle-grade readers, you can try an old classic to fit with this summer school reading theme--The Mystery of the Hidden Beach (The Boxcar Children Mysteries #41). The Aldens and their new cousin, Soo Lee, are visiting Camp Coral in Key West, Florida. But as always, a mystery pops up. The children have to investigate when a coral bed is destroyed on a nearby island. Kids will love trying to figure out the mystery right along with the Aldens.
If you are teaching a primary summer school enrichment program, there are tons of books about the beach with your students' favorite characters such as: Clifford the Big Red Dog: The Missing Beach Ball, Eloise Goes to the Beach, and Walter the Farting Dog: Banned From the Beach.