There's a mitigating factor to the sub situation: the Japanese Navy, being extremely rigid in all things, had shit for an anti-submarine service. From what I've read, there were two reasons:

1. Navy officers looked at it as extremely distasteful to escort civilian vessels--they all wanted to be Admiral Togo sailing into Tsushima in an epic, heroic battle to win the war. In any other country, they'd be told tough shit but senior officers had simultaneously cultivated and feared their underlings and relied on them for decision making far more than was healthy, and

2. Having been modeled after the British Navy circa 1910s, the Japanese Navy looked at submarine warfare as a purely military tool and thus didn't adapt until it was way too late.

From what I read, the escorts they organized had a couple of aging destoryers and cruisers mostly with overaged draftees on board, and weren't even put into service until 1943ish, at which point it was way too late. The strangulation by American subs would have happened anyway, because there's no way they could have matched the sheer number of steel ships the Americans could throw out there, but they probably could have held out much longer had they bothered with the above in 1941.