Phone is water damaged. How to fix it?

Hello,

I dropped my phone in the water accidentally. I had removed the back panel , cover and battery connector to main board. I did notice some water on main board and battery which I soaked with soaking paper.

I kept it for some time and then connected the battery after which it started normally. After some time screen was acting weird, flickering, black lines from a side.

The black patch is increasing slowly while screen is still flickering and off colored.

I had gone to service center and they diagnosed it with bad screen and main board.

I was wondering if it was bad main board then why is my device getting on ?( I can see my home page and wallpaper) I am able to charge the device all though I know that is a bad idea. How can a main board be damaged if my phone is turning on?

Can anybody help ?