disassemble iphone and clean parts and connectors with isopropyl alcohol and a soft brush. residue from water is either white or green. allow to dry, check and clean again if necessary. let it dry for a few hours to be safe, then reassemble. You can submerge the motherboard in the iso for a short time, give it a gentle jiggle to make sure iso gets everywhere and to remove the residue.
reassemble, charge on the mains charger, and let us know how you get on.
the battery is the most likely part to need replacement which is why I suggest mains instead of pc etc so you can test phone for what does and doesn't work.