Just a rejoinder..it is also to keep a vacuum in something (thereby making it more less for water to go in), it is better to just make one hole.
Imagine a can of soup or milk, punch a hole into it and tip it upside down and the contents won't spill.
Now, add another hole and it would be rather easy to displace the contents inside with air.
It is the other way around with the waterproof cameras, air is kept inside the camera so that water would not penetrate. If it just has the battery door as an opening, then it is less likely that the air would escape and let water in.
If you have two holes, apart from the battery door, then the likelihood of one of those opening failing would be more and once that happens, air would escape and would be displaced with water, thereby causing the camera not to be waterproof anymore.