Could you give some examples of these things, please? All waterproof hardware I have ever owned (phones, cameras, flashlights, watches, etc) doesn't have user-replaceable batteries. In all cases the batteries need to be replaced by a specialized shop and the waterproofness is tested afterwards in a special machine - and it's not always a success and the job needs to be redone.
Note that there is a significant difference between "water resistance" and "waterproofness".
Note that there is a significant difference between "water resistance" and "waterproofness".