Use a snake bit. But I would want a way to get in there and epoxy the board after I drilled it to keep water from splitting the plywood.
Also, put a small hole in the floor of the CC. Where the toilet sits. If water is puddling in there, it is seeping past the screws that hold the toilet and the step to the floor. The cavity below is filling with water. I pulled more than a gallon from mine. There was enough the plywood was getting spongy from being soaked. It has now dried and gotten hard again.