Well, the mines east of Formosa become active in September 1943, as do the ones in the Formosa Strait. The ones west of Formosa, along the coast of China and the 100 fathom line are Sepctember 1942. That is in the Stock game and RFB. If you use RSRDC on top of that, then the minefields might be slightly different.
You don't mention whether you are submerged or on the surface? Shore guns in Stock (RFB) are super shooters and can hit you from 9600 yards on the 2nd or 3rd shot sometimes, but those are on the west and north coast of Formosa. An airplane can see your boat though, and attack without you knowing it, if you're not too deep, or on the surface... or if submerged, maybe you smacked the bottom?? Went too deep for your boat?? RFB really makes the boats weaker in that regard, so you do have to be more careful of your crush depth with it, especially the older boats.
