My kid has a cheap waterproof watch.
Battery died, took it by a watch retailer and asked to change the battery -- they came down really hard that the watch will not be waterproof unless it is pressure tested.
I've changed batteries on my waterproof watches by myself for many years without issue, but the saleslady in the watch shop said that it was not possible. The way she was talking was that pressure testing itself somehow endows the watch with waterproofing. But it is just a test right?
Each time I changed batteries on waterproof watches, there is a little rubber gasket and you just have to be really careful that it is correctly aligned with the groove, and I sometimes put a little bit of silicon grease on it too.
Anyway, I didn't buy the battery from them. I think she was more interested in selling expensive pressure testing.
I get that if it was a really expensive watch and extensively used in the water then it might be a good idea, but good grief, the pressure test costs more than the watch.