Why did the US fleet within US littoral waters not take significant defensive action against the U-Boats in 1941/2 despite detailed intelligence from Britain's Enigma deciphering?
First of all, US was not a belligerent party throughout most of 1941.
Second, the premise is wrong: US did "take significant defensive action in 1941," and arguably offensive ones as well. Although US was not officially at war with Germany, Roosevelt signed an executive order that precluded German warships, including U-Boats, from operating within 1200 miles off US shores, lest US Navy would engage them. This is 100 times further into Atlantic than territorial waters, and contrary to international maritime laws; in fact, Roosevelt wrote to Churchill: "I cannot declare a war, but I can wage a war," referring to US Constitution that precluded him from declaring the war that US Navy was ready to engage in on the West side of the North Atlantic. Because of that German U-Boats had to retreat 1200 miles off US shores and formed a straight line in the middle of North Atlantic.