Also the fact that China is actually both signatory and ratifier of the Chemical Weapons Convention since 1997. Attempting any sort of chemical warfare against those stationed onboard the derelict, even non-lethal ones, has the potential to cause a diplomatic disaster for China.
And what would that achieve? Causing the derelict to puncture the hovercraft's skirt and stranding the hovercraft (plus the people manning the hovercraft) there? The same goes for using any ship to do the same thing.
Besides, using "it's just an accident" to cover such act is just so kindergarten-level.
Who care if the "it's just an accident" excuse can be work or not. And who care if the ship that ramming that derelict ship will be stranding on that place. The cost of a ship that you use to ram that Pinoy ship is cheap, if you compare with the headache that Pinoy can do from that derelict ship of theirs. Just ram it, make it sink, and let the new derelict ship (now belong to China) fill the void, to become the flag, to show that the rift belong to China, now.
The cost of a ship can't be compare with the symbol that Pinoy can use, to declare that the rift belong to them.