I had 2 destroyers and an 2 air with the second carrier sink actually. I didn't return you and you rolled low, you did get lucky.
But a 1 in 6 chance is great odds against the japanese because of their pitiful one hit carriers. The problem wasn't the subs in this instance, it was the carriers. A 1 in 6 chance against the japanese means that about every 6 shots, you should sink a fully loaded carrier which means you destroy 36 points worth of navy or about 6 pts a turn, not to mention time lost building another carrier and planes. This is better than doing economic damage with them.
Subs have to be the best piece in the game, I am just glad that both sides can build them.