Galaxy S8 moisture problem can occur for any phones, even the phone is not in contact with water!
When you have the moisture problem in Galaxy S8 or S8+, you cannot charge the phone through USB cable.
This Galaxy S8 how-to guide explains the causes of Galaxy S8 moisture problem and shows you the solution for such a moisture problem in Galaxy S8 and S8+.
What is Galaxy S8 moisture problem? Isn’t Galaxy S8 waterproofed?
If you Galaxy S8 or S8+ was in contact with water, the USB port (multipurpose USB Type C port) in the bottom (see Galaxy S8 and S8+ layout) may be wetted.
Samsung equipped Galaxy S8 and S8+ with a humidity/moisture sensor to detect moisture buildup in the charging port.
Once the sensor detects the moisture is higher than a preset level, it will believe you get the Galaxy S8 moisture problem.
When you try to connect this Galaxy S8 or S8+ with moisture problem to the charger, the phone will refuse to charge the battery and you will get a warning as shown below:
Moisture has been detected. To charge your device, make sure your charger/USB port is dry.
Preventing Galaxy S8 or S8+ from charging when moisture is detected in the USB port is a preventative measure to prevent more corrosion.
It is also a security measure.
Please note, Galaxy S8 moisture problem has nothing to do with waterproofing feature in Galaxy S8 and S8+.
The USB port is not sealed in Galaxy S8 and S8+. It is safe even if the USB port is immersed in water (less than 1.5m).
But water, especially seawater or swimming pool water may cause considerable corrosion if you do not rinse and dry the port timely.