Buddy diving just before and after the Second World War. After the invention of the "aqualung" by Cousteau and Gagnan, the first commercially underwater breathing apparatus became available for sale for sporting purposes in the late 1940s. As the new sport of scuba diving rapidly expanded through the 1950s, several sporting organisations—notably the YMCA—began programmes to train swimming enthusiasts in this new aquatic pastime, and began to codify what they believed were proper practises for this expanding sport. The YMCA considered the buddy system a useful corollary to the "never swim alone" rule of their swimming and lifesaving programmes. Cousteau himself independently
The Voyage of the Arctic Tern Rose, which was an example of a carrack warship. A bit of the Mary Rose wreck's recent history is mentioned in the story, with a brief description of its first salvage attempt in the 19th century by brothers John and Charles Deane. Other real historical figures are also briefly mentioned in the story, such as 19th century French microbiologist Louis Pasteur, and 20th century French co-developers of the Aqua-Lung, Émile Gagnan and Jacques Cousteau. The main fictional character of Bruno was inspired by a real-life diver of wrecks named Bruno, whom Montgomery had met while working on the Mary Rose