iPhone only works when connected to charger (water damage)

Anton, the very first thing that you want to do is to disassemble your phone, use these guides, and clean everything with a high grade isopropyl alcohol, do not forget the connectors. Use a soft brush to clean it with as well. Check for any missing, burned, or otherwise damaged components. Carefully check every part for corrosion. Do not forget the connectors on the board and the cables as well. Once cleaned, replace the battery. This is not an option but a must. Remember that the batteries are smart batteries. They do a lot more than just store energy:-) Re-assemble your phone and re-evaluate for further, if any damage. Hope this helps, good luck.