My iPhone 6 Plus won’t turn on

You most likely have a short somewhere on the board. You need to hook it to a bench power supply and see if it is drawing any current without being powered on. If it is then your primary short is going to be on a main power line (likely vcc_main). A favorite capacitor to blow with water damage is the one on the back of the board near the wifi chip. It is very close to the sim slot which is why it blows typically with water damage. You will need microsoldering equipment to remove and replace this component. You can remove it and see if your short is cleared. Your phone will operate without this capacitor but if you plan to keep using it I would recommend you simply remove and replace the component.