I don't trust it at all.
The coating has gaps in it (as far as I could tell when opening my Kindle Paperwhite 4), and if water got in it'd probably leave residue and whatnot. Not to mention it would take ages to dry.
The original H2O water seal breaks easily (happened to my mother's H2O when snapping it into Fintie case, the front bezel was lifted off in one corner) and for the flap it's hard to tell whether it's closed properly.
There may also be long term effects, a German reviewer (allesebook) put their H2O in the freezer and it was fine afterwards, until it stopped working half a year later due to rusted contacts inside. Water and electronics just doesn't mix. So, I keep my devices away from water. If in doubt, bring a ziploc bag or similar.
That said, some people manage to drown their non-waterproof devices and have them survive regardless (after removing battery and allowing to dry for a week). So maybe having even just a little protection is already considered enough in most cases?
Even so, I just don't trust it. And if I could choose between a waterproof device and one with microsd slot, I'd always go for the microsd... user-replaceable storage is super useful (especially if you're into modding), waterproof is just a gimmick...
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