In these routine but readable WW II submarine adventures, the only novelty is the submersion of Britain's midget four-man subs, which were used to distract the Nazis in Norway from the D-Day invasion buildup in 1944. The action begins when the Germans hide their new rocket on a ship in a fjord--but also on board are many Norwegian prisoners. The X-16 minisub's commander, Lt. David Seaton, swallows his humanism and blasts the ship to the bottom. His second assignment is to return to that very ship, recover a secret device from it, and pick up a defecting scientist. Seaton emerges thoroughly scraped but not scrapped. His third mission: blow up an underwater U-boat pen and rocket-launching site on the French coast. The two love stories back on shore are sugar-free, as is the stout, stolid tale-spinning under the frigid North Atlantic.