There is a small hole on the inside of the engine that is used to fill the reservoir when assembling (at least on pre '97 motors). But the gears themselves are surrounded by small gaps of air so they can move freely. This would likely account for most of the water penetration.
The counterbalance oil just sits at the bottom of the chamber and as the gear spins it gets continuously coated in oil. The chamber is designed to hold the oil even if upside down as well as when you flip your ski over (as long as you flip it in the direction your rear sticker tells you).
But if the ski floods, you should check this reservoir just to be sure your oil is good. If not, you should change it. This sucks for the pre '97 engine, as you do not have easy access at all. One option you have is to drill and tap a hole in any 787/800 engine without it, but who wants to do this unless your engine is already out.
They definitely saw the need for it in the later engines, but... to put those at ease who are thinking right now "oh shite!"...we hear a lot around here about engines flooding but very little of holes being blown out of crankcases. It would seem to me that the severe damage is a more unlikely scenario. And I would bet that less than 10% of those people are changing their counterbalance oil.