The same way you do with anything else that's waterproof and has removable batteries....You have a small rubber gasket.
Note that there is a significant difference between "water resistance" and "waterproofness".
A few companies have business/military phones that still have most of the features people were used to. They are a little bigger, but you don't need a case.
Rugged 2-way radios, not just waterproof, but designed to float and to turn on the lights when dropped into water. Perfectly replaceable battery pack, interchangeable with the regular NiMH batteries. All it takes is literally rubber inserts that you need to pop off to access the screws.