Luckily that phone is IP68 dust and water resistant. Salt water can be highly corrosive and probably caused corrosion in the port, causing a short circuit which results in that message and stopping the phone from charging. You can use a toothbrush and some compressed air to clean it out. If you have something like a pipette you can use that to drop a little rubbing alcohol in there to help clean it. Just be sure to power the phone off when you do so.