With a water damaged phone, the first thing to do is a proper decontamination (visual inspection,remove shields, ultrasonic bath with proper solution, rinse in IPA, air dry...). Then you start troubleshooting, focussing on the areas that had the most damage.
Water damaged phones usually end up being very difficult to repair. If you are experienced in probing a logic board, then I would check the main voltages (PP_VCC_MAIN, PP_BATT_VCC) and then start checking the PMIC output voltages to see if anything is amiss. Finding bad or missing voltages may help you isolate what part of the board is damaged.
If you are uncomfortable doing this then look for a shop that specializes in water damage and micro-soldering repairs.
Repairing iPhone Liquid Damage