Probably one of 2 IC chip may have problems. On the back of the pcb below the black tape there are 2 IC
located over the Large Metallic chip your PMIC in this case. Above the PMIC you have your Mason IC and your Cumulus both those IC controls touch and display to your phone. If you are not experiencing any zebra lines than most likely the mason chip is bad the black chip over the small metallic BCM chip the on the back to the left of the board the number on it will start with 343S. The only way to repair is to replace the bad IC. This is something that take special tools and knowledge to repair. Search your area for a shop that performs this type of repair. Another possibility is that the screen it self may have gone bad due to the water damage.
Although you have a display the tough on the screen may have gone bad. This is an advance issue and needs some one with a little knowledge to diagnose the correct problem.