iPhone 6 won't work unless plugged in
From your question I'm assuming the iPhone battery worked well prior to being dropped in the toilet.

It looks like you need to have the battery replaced.  If you have AppleCare, you should go to and Apple Store or contact Apple support to have the battery properly assessed and, if needed, replaced.  See this Apple page for info on replacing the battery.  I would recommend going this route as it appears to be the lowest cost option.

If you don't have AppleCare, you can go to any 3rd party that you trust to replace the battery.  You can also go to Apple to pay to have them replace it.

If you're handy, you can also replace the battery yourself.