Why weren't there reverse depth charges?

This is true except for the Japanese, who saw theirsubs as, basically just another form of warship. Their subs were by far the least effective of the major power.

On second thought, I should probably take that last bit back. I was referring to Germany, the US and Japan, and there were several other countries using them but I don’t know much about their policies, success rate, etc. Italy, Russia, and England all used subs, and maybe some more too.

While this is an accurate description overall, I will nitpick your speeds – they’re a little too high. Wikipedia puts the max submerged speed of a Type VII U-boat at 7.6 kts, and fast convoys at 9-13 kts, slow convoys at 8 kts. 15-knot ships generally sailed independently and were considered very fast. They existed, but I wouldn’t call them average.

Otherwise you’re dead on.