I often read in the bath, as it's a common way for me to relax. Kobo Libra 2 has an
IPX8 rating, which means it can handle being submersed under water deeper than 1 meter for more than an hour. Or at least that it has been tested once to be able to do so.
I've defined waterproofing in my original post as protection against being briefly submersed in water, so usually IPX8 and IPX7 ratings are good enough for that purpose to give me enough peace of mind. IP classifications are not industry standards, but they are the closest we have and there's at least some testing involved.
The word "waterproof" is often used in marketing and in common parlance it means that a device will not let water inside its casing for some period of time. Similarly, "water resistant" usually means that there is some protection against casual spills and splashes and "water repellent" usually indicates that there is some sort of a coating over the electronics that repels water. None of these mean that a device is safe from all water for all time, just that there is some degree of protection against it with an unknown degree of durability and reliability.