The Treasure of San Bosco Reef.

The story concerns young David Jones, who travels to Italy during his vacation to work with his uncle, Max, an archaeologist who raises ancient artifacts from the ocean floor. One of Max's divers is killed and the site becomes suspect--especially after several of the artifacts disappear; they know these will be sold illegally to rich collectors. When word gets out that these items have disappeared, other men are endangered on the site, and, indeed, someone tries to kill Dave. Helping Dave and his uncle is a young Italian boy named Augusto Donato, who tells Dave to call him "Gus." Dave teaches him to dive and in return Gus helps Dave spy on several of the suspects and also helps save his life when Dave is attacked.