When a phone comes in contact with water it is best to disassemble the phone as soon as possible and remove the battery. Next you should clean the logic board and other parts such as cable connections with 90 percent or higher isopropyl alcohol and a soft bristle brush such as a toothbrush. This helps remove minerals that are left behind when water dries. If not cleaned the minerals may lead to corrosion and possibly shorting of electrical components. If the battery came in contact with water it may need replaced. After cleaning with isopropyl alcohol try reassembling the phone and test if it powers on and functions correctly.

If you still have display issues after cleaning with the isopropyl alcohol then the display is probably damaged and you will likely have to replace the display.

There are many instructional guides on YouTube that can show you how to clean a phone with isopropyl alcohol, and some guides on this site too. Good luck and let us know if it fixes the issue or if you need more help.