iPhone dropped in water!

Yuqii, let me guess. You never cleaned it after the water damage. Now, the best thing I can suggest to you, is that you disassemble your phone using this guide. Then, clean your phone and follow this guide. Make sure that you clean it extremely well, since it is possible that you are already having corrosion issues. Once you clean the phone (double. no triple check that you cleaned it well) replace the battery. After you cleaned it and replaced the battery, re--evaluate and see what damage, if any, you have. Hope this helps, good luck.