Having the same issue :/ Started a couple of days ago. No contact to water or anything.
However it will let me charge if the cable is plugged in while I restarted the phone. But if I unplug it and plug it in afterwards or just gives me the same message again :/ So for me restarting the phone while the cable is plugged in for charging works.
This is exactly what happened to my son's. AT&T refused to help me with it; claiming it's my fault for not having insurance. The phone was bought on December. My son has been grounded and his phone in my room, charging in a drawer rarely used. I woke up at 4 am to the warning and the phone eventually died with no help. So frustrated right now.
Samsung advertises it's "waterproof." If it was then there wouldn't be this issue. When I confronted the Samsung rep on the phone she in fact did balk at the fact that they are waterproof. Came up with several excuses which center down to one thing and these were her words "fresh waterproof only but even that is limited." Also admitted that there could be a software issue. Sounds like FALSE advertisement to me. My box says waterproof and dust proof and something about being able to be submerged for around 30 minutes. Does NOT say anything about "fresh water only."
Samsung advertises it's "water resistant" ask anyone. No such thing as "waterproof" even military grade submarines can't make to the bottom of our deepest oceans there for they are water resistant not waterproof.