This is one of those things that you have to trust your instincts on. Add salt when water feels like it makes you more thirsty. Don't, when it feels good by itself. If you add too much your taste buds will say "gross!". Look for that juuuust right place. I pretty much add salt to my water all day, although sometimes I don't want it.
Again, if you are getting up to pee at night, I think
it is important to add salt to the water that you drink before bed.
And yes, you can drink too much water when you have AF. Tune up those listening ears. You can crash your sugars and salt by drinking too much.
Oh, how fun will be the day when just drinking water by itself is satisfying.... Sigh.
At least, of all the AF sypmtoms, this seems the easiest to rectify, at least in my opinion it is..., once you get the hang of it, of course.