Well ok if you want an explanation, you are correct, it does kind of a poor job at keeping the water out. The basic engineering thinking on the seal is that it is there to keep the oil in. If the case is full of oil, the water can't get in. Yes, a small amount of water can get in anyway, but that's why the owner's manual says to change the oil as a periodic maintenance item. Better than the real old motors where you had to re-lube it every time you used it. The back to back arrangement on modern motors is even better.