Battery wont charge but the phone turns on after water

First and foremost if you have not gotten someone to ultrasonically clean your phone, do it before you go any further. Water damage is very tricky. More than likely there is nothing wrong with your battery although I do always replace batteries on water damaged phones. The water probably damaged the charging IC or there could be another circuit that draws current from your battery to your phone that is damaged. The only way to truly know is to take it to someone who does micro soldering repairs. Have them ultrasonically clean it and find out what chips need to be replaced.

You could always try cleaning the phone circuits and connectors yourself with 91% or higher isopropyl alcohol and an antistatic brush (or a tooth brush if you have nothing else). Do not do this if you are not comfortable disassembling AND reassembling the entire phone. Either way, fix it or get it fixed soon. The longer water sits inside a phone, the more corrosion builds up. Anyways now if you still feel comfortable doing it try this! Look for green corroded solder joints and scrub that junk down with alcohol! You do not have access to scrub every part of the main logic board (there are plates soldered on over it), but at least clean out the board connectors, flex cable connectors and every part of the board that you have access to. Sometimes the corrosion that happens from water damage causes pads to lift and connections to be faulty. You should replace your battery after water damage. If you are already opening the phone up, the battery is easy and cheap to change. I have had success with this method on numerous occasions at the shop I work at. Just wanted to give you the OPTION to try and fix it yourself(thats what ifixit is for). If your phone still does not work after that consider taking it to a microsoldering specialist as the water has likely damaged a component beyond repair, or has spread underneath the metal plates I was talking about earlier.