The ring is rated as IP68 so that means you can wash your hands with it or go swim in the pool in fresh water but you shouldn't use it for dives or anything related to the sea. It also gives it dust resistance because of how it is enclosed.

It depends on multiple things to be honest, the most important of which being the specific phone type you are using and how thick the case actually is. If it's a normal case on a normal phone, then it should work well, but if the phone doesn't have a great NFC reader, is maybe a bit old as well and you also have a very thick case on it then yes you might have some problems getting the phone reader to see it; this is somewhat normal and expected behaviour to be honest, however the worst that can happen is you just have to move it around and hold it there for a bit longer so the reader can recognise it so it shouldn't be the end of the world.