It is the opening at the bottom on the near side bumper just over the splitter. The air box is actually fitted with water release flaps on the back but they may as well not be there for the good they will do!
I'd suggest that the moment the splitter is in the water you will scoop up water into the air box, the bottom of the stock air filter is resting on the floor of the box and being paper will mop up even small amounts, it will very quickly become overwhelmed and as the engine is acting like a great big pump you are in trouble before you know it!
Best thing I did to my 4200 was rip out the air box and go K&N in the wheel arch up behind the headlight. At least it could breath and not drown.
The GS is awaiting similar treatment I just haven#'t needed to get the bumper off yet.