Mar, getting hot after water damage is never a good sign. You did not tell us if they cleaned the board. The best thing to do is to stop using the device, (remove the battery) and to totally disassemble it. Here are the guides to help you with that. Use some sterile water and rinse the parts off to remove the sale. Then clean it with +90% isopropyl alcohol and a soft brush.. This will displace the water and stop corrosion. Use this guide to get it cleaned. It is written for a 3G but applies to your phone as well. It is okay to submerge a board in isopropyl alcohol. After that, air dry all the components. Do not forget to clean the connectors on the board as well as the cables. Then re-assemble with a new battery This is not optional but a must to avoid failure later on. Check your board for any missing or damaged component. Water damage is the toughest to trouble shoot and to repair. Hope this helps, good luck.