Its a service port.
I think it is meant for opening and adding a little oil or grease to the Gearbox.
To much oil and it will run out of an unsealed reel through bearings and other gaps and cracks .
To much grease and it will work its way up the pinion/drive shaft cavity areas and into the ARB causing malfunction.
Adding these through the port can be done with a loaded syringe.
But - Yes, if the box is filled with water you could open the port and drain it via the service hole.