I'm having the exact same issue and it's with my second phone. I used to have this happen on my original S8+, I cracked the screen, got a new one, and the new phone has the same problem. Was not in contact with any water, but won't let me charge whatsoever, on or off, and keeps saying moisture detected.
I've tried using blow dryers, my car's AC, napkins to try and get it to "dry", but nothing. What's the point of having a phone that won't let you charge and is just dead for days at a time?
Volume down to reboot cell... works.. but message has been reoccurring. . Never been in water.. humidity.. nothing. Samsung is really not cooperative.. If it goes on too long I'm going back to my note 4
this is what i did by a freak chance and it seems to be working in the mean time.
1. plug in your s8 to the charger (you will be notified that there is moisture detected)
2. restart your s8 (make sure not to remove the charging cable)
3. make sure it shows how long until full charge.
4. You have by successfully by passed the moisture error.