Dead Wake: The Last Crossing of the Lusitania

by Erik Larson

The most important effect of all this was to leave the determination as to which ships were to be spared, which to be sunk, to the discretion of individual U-boat commanders. Thus a lone submarine captain, typically a young man in his twenties or thirties, ambitious, driven to accumulate as much sunk tonnage as possible, far from his base and unable to make wireless contact with superiors, his vision limited to the small and distant view afforded by a periscope, now held the power to make a mistake that could change the outcome of the entire war.

U-boats in fact traveled underwater as little as possible, typically only in extreme weather or when attacking ships or dodging destroyers.