Ittl, the Germans have invested a lot of material in China and one of the beneficiaries is the Chinese naval armament program, where rebuilt shipyards churn out submarines and cruisers.
Japan invades around the same time as in otl, but China has way more industry and professionally trained and equipped divisions and turns the war into a stalemate.
My question is, at this point, how much damage to the Japanese navy could a hypothetical Chinese submarine power do? Would they be able to sink warships or would they be more effective as strictly convoy raiders?