You cannot claim innocent passage from open sea going into an inland sea or marginal sea. You can only claim innocent passage when you transit between 2 open sea. That's what innocent passage means, which mean you cannot avoid but gone thru someone's territorial water in order to transit thru 2 open sea.
South China Sea is an Inland sea, which mean they did not connect any open sea (ie a dead end), hence any navigation into the inland sea cannot be see as "innocent" as oppose to you are entering with an intention. The only circumstance when you can claim "innocent" passage in an inland sea is when you enter the area by navigational error.
Much like you are driving a car, you will not intentionally go thru a dead end to get to another place (Which it does not make sense unless you want to punch thru the wall or house and get to the other side of a dead end street), you can stray into the dead end street if you are lost, read the map wrong or follow a bad instruction of a GPS unit, that would be innocent.
P.S. I am not interested in getting into China do what and US do what chest thumping, I am just explaining to you what does it mean by innocent passage as per UNCLOS