From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 591-594, 611:
The quality of life on American submarines was greatly improved by 1944. Occasional showers were now possible, and rations were dramatically better. The captain of one sub reported that “our freezer was filled with boned meats—including steaks, roasts, chops, and hamburgers. The baker was up at 0300 each day to prepare fresh breads, rolls, cakes, and cookies.” On most subs, there was an “open door policy” that allowed crewmen to help themselves to cold cuts and sandwiches as well as fresh coffee around the clock. A number of boats had self-service Coca-Cola machines, which one skipper called “a real morale booster.” Periodically, the crews might gather in the forward torpedo room to watch a movie. Such luxuries were unimaginable to the crews of Germany’s “iron coffins,” or, indeed, those of Japanese or British submarines.
The new American subs were also more efficient. The torpedo problems had been largely solved (though the loss of the Tang showed that some problems remained), and the number of Japanese ships sunk increased dramatically. Whereas in 1942, American submarines sank a total of 612,039 tons of shipping, in 1944 they destroyed 2,388,709 tons, nearly four times as much. If that was less than the tonnage claimed by Dönitz’s more numerous U-boats back in the “happy time” of 1942, as a percentage of Japanese shipping it was far greater. In 1941 the Japanese had nearly 6.4 million tons of merchant shipping. Despite adding 3.5 million more during the war—nearly half of it in 1944—by the end of that year there was less than 2.5 million tons left. The Japanese merchant marine was steadily disappearing because Japan could not do what the United States did: build ships as fast or faster than its enemy could sink them.
Another reason for American success was that Japanese anti-submarine warfare was not particularly effective. Japanese escorts had both sonar and depth charges, but their crews were less efficient in using them than the British in the Atlantic or the Americans in the Pacific. It was not uncommon for American subs to endure prolonged depth charge attacks with little or no damage…. Of course, having to lie quiet and endure a depth-charge attack, even an unsuccessful one, was psychologically draining. The repeated concussions often shattered lightbulbs and loosened the cork lining on the bulkheads; still, as long as the pressure hull held, the boat survived. Japanese inefficiency in depth-charge attacks is especially curious since they were extraordinarily efficient in most other areas of naval warfare. The explanation may be at least partly cultural. Valuing the offense over the defense, Japanese destroyermen worked harder at perfecting torpedo attacks than they did at the more pedantic job of escorting lumbering merchant ships or pinpointing the location of unseen American submarines.
In addition to the gradual depletion of the number of Japanese ships, those that survived became increasingly inefficient. One reason was a shortage of cargo handlers. By 1944, conscription had swept up most experienced longshoremen into the armed forces and Japan was compelled to rely on dock workers rounded up from the regions they had conquered—Filipinos, Koreans, and Chinese—as well as Japanese women and even American prisoners of war. Such workers were inexperienced, and many of them were less than enthusiastic in their labor, so efficiency suffered. Another problem was Japanese reluctance to embrace convoys. They did not put a convoy system in place until late in 1943, and convoys did not become routine until the spring of 1944. Even then, there were so few escorts that convoys were delayed, sometimes for weeks, for lack of an escort vessel. In such circumstances, it seemed wiser to send out ships individually, especially through what were assumed to be safe areas. The problem was that by 1944 there were no safe areas.
The firebombing of Japan’s major cities was apocalyptic. The postwar Strategic Bombing Survey concluded that “some 40 percent of the built up area of the 66 cities attacked was destroyed. Approximately 30 percent of the entire urban population of Japan lost their homes and many their possessions.” The impact that such devastation had on Japan’s wartime economy is less clear. At the time, the [Army Air Forces] insisted that destruction of the “housing units” of factory workers weakened Japanese industry. Yet most of the industries in the areas that were destroyed by firebombing had ceased to function long before the raids began because American submarines had halted the delivery of most raw materials. A factory without access to raw materials is just a building. Several of the air strikes directed at Japan’s petroleum resources, for example, hit refineries that were no longer functioning and tank farms that were empty. The historian Mark Parillo put it anatomically: “The submarine had stopped Japan’s industrial heart from beating by severing its arteries and it did so well before the bomber ruptured the organ.” Given that, the B-29 firebombing raids that began in March 1945 and continued almost without interruption for the rest of the war were less strategic bombing than terror bombing.