This was in an article on the NY Times this morning about AI and the rules of war. http://mobile.nytimes.com/2016/10/26/us/pentagon-artificial-intelligence-terminator.html
Then came the Japanese attack on Pearl Harbor on Dec. 7, 1941. That took just six hours for the United States military to disregard decades of legal and ethical norms and order unrestricted submarine warfare against Japan. American submarines went on to devastate Japan’s civilian merchant fleet during World War II, in a campaign that was later acknowledged to be tantamount to a war crime.
I've never heard about this aspect of the Pacific War.
So that is a bit of an ambiguous statement because it could be interpreted two different ways.
The first is that, yes the USN practiced unrestricted submarine warfare, complete with total blockade and mining of the Japanese empire. The effort was successful in ways that Donitz and the Kriegsmarine could have only dreamed of. Sinking more tonnage than any other allied branch.
However that did raise some unfortunate comparisons at Nuremberg. In particular Donitz when charged with war crimes for waging and planning a total submarine war pointed it out in his defense, and Nimitz himself wrote to the effect that it was a similar strategy as to Allied efforts. This did have an impact on his convictions and sentencing, he did not get off completely but compared to similarly senior officials he got off pretty light.
The other factor of specific incidents of what might lead to a war crimes trial in the USN submarine campaign, yes we do find them from time to time. The most famous of which involved the famed USS Wahoo on January 26, 1943, skipper "Mush" Morton was claimed to have ordered the deck guns and crew to open fire on survivors in life rafts of a Japanese transport which he had previously torpedoed. While the event is still unclear, in part because only one side had any survivors, there was a consistent pattern on both sides of remarkable cruelty to recovered ship or plane wreck victims and resistance to rescue from Japanese sailors. While it should also be noted that the Hague Convention explicitly protects shipwreck survivors from being killed without provocation, and a U-Boat skipper was tried and executed for a similar incident. Morton faced no reprimand, certainly in part because he was killed several months later with his boat on patrol, earning a hero's death. The primary account of the incident then comes from his XO at the time Dick O'Kane who was arguably the best submarine skipper of the war(with Morton certainly close behind in efficiency) and who earned a Medal of Honor for his efforts in command of USS Tang.
For a good intro to the USN submarine campaign I would recommend Ian W. Toll's ongoing trilogy on the Pacific War, the second volume of which Conquering Tide features Morton and O'Kane in particular.