Default [Q] Help regarding water damage in Xperia M

Hi XDA,

My xperia M accidentally fell into water last week, as a precautionary measure I removed the battery and left it as it is till evening and in the night when I put the battery again in its place, it started working fine. I used it flawlessly for the next 3-4 days until I started getting rebooting issue whenever I plugged in the charger now.
I went to the service center and first they updated the software and said its working fine now but again when I plugged in the charger I ran into the same issue.
Now the battery is getting drained very soon and charging it for hours too doesn't help.
Rebooting problem is still there, sometimes instant reboot, sometimes within minutes.
The guys at the service center are saying that the motherboard needs to be replaced.
I would like the Senior members to suggest me whether should I go for a motherboard replacement or is thr any other way to get this fixed.

Plz Help.