I've spent the summer reading The Stories of John Cheever, mostly on the beach. Some other memorable beach-reading experiences:
Ayn Rand's The Fountainhead: When I was seventeen, I was really taken in by the whole objectivism thing, as most impressionable youths are, I guess. Thankfully, my critical thinking skills improved in college.
Vincent Bugliosi's Helter Skelter. I read this sometime in college, maybe the summer between my freshman and sophomore years. I could only read it in the daylight because it scared the crap out of me.
May Sarton's A Shower of Summer Days. Another summer read during college. My memory is a little hazy on this one, although I do remember subtle lesbian undertones.
William Somerset Maughan's The Moon and Sixpence: Loosely based on the painter Gaughan's life. Not technically a beach read, but I read it when visiting Put-in-Bay, Ohio, one of the little islands off Lake Erie.
Katherine Anne Porter's Oh Immortality: I read this last summer while visiting friends in Myrtle Beach.