Great question! I should probably write more about this at some point. As I mentioned the Mediterranean and Baltics are special. It gives you a relatively calm sea which connects a very large landmass. The South China Sea is entirely open. I am not a maritime expert, but I am pretty sure the South China sea has much rougher weather conditions. The Mediterranean offer much safer conditions since it is enclosed. Early ship navigation relied strongly on being close to shore. You can sail pretty much anywhere in the Mediterranean while following the shore.

In the South China sea you sea much larger open oceans that travelers would have to deal with.

I would say Korea and Japan fit much closer the ideal I am talking about. Both have quite a lot of coastline relative to landmass and the sea travel is much shorter. In the South China sea the distance are quite significant.

Also as I have elaborated on in linked articles, climate is crucial. The countries you mention are much too far south. The means things like rain seasons and significant challenges in managing water flows. To have waterways which can be exploited for things like waterwheels you need much cooler climate. Even Japan and Korea are probably a bit too hot.

A place like Britain the steady rain through the whole year keeps rivers stable and navigable through most of the year. Close to equator you have much more extreme weather which will cause major problems for yearly commerce and manufacture.