Dive in the Sun — Douglas Reeman

The Allies are poised for the invasion of Italy. Yet the Germans hold a vital card: a floating dock, the only one in the Adriatic large enough to take a major warship. Moored at a small port near Rimini, it is exposed, vulnerable. It must be destroyed before the Germans can tow it to a safer harbour. Air, surface, and conventional submarine attacks are out of the question. Only one team can do the job and still stand a chance of surviving -- the crew of His Majesty's midget submarine XE.51.