iPhone XS water damage screen or motherboard?

Long story short I did something silly used my phone in the swimming pool to take a picture, the phone is working but the screen isn’t turning on. a Repair shop has offered to fix the phone granted its a water damaged motherboard, but it could also be the OLED. SO, in short, what could it be the screen itself or the motherboard? again the phone works turns on and off and the buttons also work, however, the screen was working, then it turned green, it then started flashing green and now it no longer lights up. If anyone could help that’d be much appreciated. thanks.