The first of a trilogy (the other books are Undersea Quest and Undersea City) of SF "juveniles", is in the fine tradition of Heinlein's early works for younger readers that seem to have set the tone for this type of science-fiction novel. Written in the mid-50s, the books' setting was probably inspired by the voyage of the nuclear-powered US submarine "Nautilus". It's a world in which the oceans are dotted with large underwater domed cities and undersea vessels ply the traderoutes between them or engage in harvesting and mining the vast wealth of the sea.
Our young hero is James Eden a cadet at the Sub-sea Academy, the service school for the undersea fleet. The Edens are a family famous in the realm of undersea exploration, since his uncle, Stewart invented "edenite", the material that allows ships and structures to withstand gigantic underwater pressures. The Edens have their rivals, once of which figures prominently in this book.
In a spare 150 pages, James has to outwit his enemies, solve the mystery of his uncle's disappearance, and try to keep himself from being tossed out the academy. It's pure and simple fun, an engagingly written (I sense more of Pohl at work than Williamson) adventure for young adults or older readers who want to capture a bit of that nostaglia that comes from the type of books they read when they were young.