What do I mean? In hardware manufacturing lines, if you guarantee water immersion, you actually do that test as part of manufacturing, either using real water or something known as "dryleak" where you gauge air displacement in a fitted and known volume fixture.

Apple and most phone manufacturers aren't going bother with that test since it's extra labor and time, so they instead live with never warranting it.

It doesn't take much to have a water ingress issue. Even a tiny tiny defect in the adhesive gasket gluing the phone together can lead to ingress.