On My Honor by Marion Dane Bauer, 1986

Twelve-year old Tony makes plans to climb the dangerous Starved Rock bluffs with his best friend, Joel. Joel's reluctance to do such a foolhardy thing is satisfied when Tony's attention is turned toward swimming in the Vermillion River. After some goading, Joel dares Tony to swim out to a sand bar. Joel makes it. Tony doesn't. Highly recommended.