Gday Pigg, the iPhone 8 isn't waterproof to begin with, Its only IP67. Using canned air on a wet phone could actually force liquid in, especially if it had enough pressure to inflate the phone. Air should escape/enter the phone easier than liquid. I would avoid taking your phone in the shower... use that 10 mins for yourself. All the best mate.
There are a lot of separate issues here, i would recommend wiping the phone and restoring it. Make sure you are on the latest iOS. At least then you will be able to eliminate the chance of a software fault. You would be surprised how often this works.
Sounds like a faulty battery or charger. Try a new cable, plug and power point. If still no luck try a new battery. Worst case would be the battery connector on the logicboard may need repair/replacement. Best of luck!