How do I know what is the damaged part?

India, the problem with water damage is that you most often do not see any visual damage. It is usually caused by circuits that have short circuited. I suggest a thorough cleaning with a high grade of isopropyl alcohol. That includes the connectors and cable ends. Yes, you submerge the logic boards It displaces any water and removes most of the contaminants. After that, always replace the battery. Clean it just the way shown on here for the iPhone. Hope this helps, good luck.