ON MY HONOR, by Marion Dane Bauer (Written by Cireana)

On my Honor is about these kids who go swimming.  This kid named Tony wants to go climb the bluffs at first, but Joel tells him that a kid died from climbing them.  Joel wants to go swimming in the pool instead, but Tony convinces him to ride to the bluffs anyway.  On the way there, Tony decides to go swimming in the river.  Joel is scared, but he goes anyway and they race to a sandbar.  While they're racing, Tony disappears.  To find out what happens to Tony, you'll have to read the book.