It was a decision based on all known information plus a life-time of deep-sea experience.

From The U-Boat War 1914-1918 by Edwyn A Gray.  Page 15.  On the sinking of the Lusitania.  

Groping beneath the surface at less than 5 knots U-20 was rapidly losing ground.

But, despite Schwieger’s fears, the Master of the passenger ship, Captain Turner, was in no great hurry. He was already in receipt of a general warning signal from the Port Admiral at Queenstown that there had been submarine activity in the area and, just before noon, he received a further message that U-boats were in his immediate vicinity. Swinging in a wide arc to clear the Fastnet, where he thought the enemy might be lying in ambush, Turner pushed on into the St George’s Channel. And, as a precaution, he ‘had all boats swung when we came into the danger zone.’

As the Old Head of Kinsale came into view he rang down to the engine rooms for a reduction in speed from 21 to 18 knots. As he explained later: ‘My reason for going eighteen knots was that I wanted to arrive at Liverpool bar without stopping, and within two or three hours of high water.’

It was a decision based on all known information plus a life-time of deep-sea experience. According to the Admiralty warning signals the U-boats, if there were any, were well astern by now and, in any case, his speed of 18 knots was amply sufficient to outpace any stray submarines he might encounter. Unfortunately the decision, made correctly and in all good faith, was to cost the lives of 1,198 innocent men, women and children.

Decision making in complex or chaotic systems is a balancing of probabilities tightly constrained by incomplete information.  The best decision can still be wrong.