If I were to guess, I think you would need a minimum of 55-60m water depth for an SSBN to hide in without risking collision with ships or the sea floor. 20m for the draft of the largest cargo/tanker ships, 25m for the height of the SSBN and maybe 5m above and below the SSBN for safety margin. That's 55m minimum. That rules out Bohai Sea, Yellow Sea, and half of East China Sea. An East China Sea launch is need for the JL-2 to hit targets in Alaska and Hawaii, but this area is fraught with risk given the proximity of US and Japanese bases in the area. From the South China Sea you need a range of ~9,600km, which is what I'm guessing is going to be the minimum range of the JL-3. To hit bases in Southern California and the Minuteman silos in WY, MT, and ND from the SCS you need ~12,600km.
You would think it would be easy; just listen for their noise. But that doesn't explain all the incidents at sea where subs decide to surface only to ram into ships as they ascend. Plus there is going to be a certain minimum launch depth for the SLBMs.Would it be possible just to avoid the largest cargo ships (with 20m draft) ... I don't think it is difficult task