Yes, it was hard to put back in! I thought it was just me. So you saw the plug on the other side too? I was thinking I'd pull that one as it is a little lower, but it also looks like it may not be a plug so I left it alone. Looks like it is on the side of the bearing housing for the driveshaft, so I thought it may help hold that bearing in place. I'm not too worried about water in the clutch housing - just a little curious.
I have a couple of old Farmall tractors, and with them, it's actually possible to drain a gallon of water out of the rear end if it has sat for a while! Most people think this is due to condensation inside the castings, but there are also a couple other paths for water to make its way inside - around bolt heads, etc... A little hard work heats things up and burns off the water if there isn't too much, but if it has sat for a year or so with light use, you just drain it off.