I made a similar boneheaded mistake last week. I hopped in the hot tub for 5 minutes before I remembered my four-month-old AT&T Galaxy S3 was in my pocket.
When I noticed, I imediately hopped out, pulled the battery and dried it off as well as I could. I patted every opening and button with a paper towel until it stoped getting damp. Then, I put it in rice for a couple of hours.
Initially the screen powered up with a purple hue and one horizontal pink line 1/3 of the way up the display. A few hours later it lit up perfectly with the SAMSUNG launch screen, followed by the Galaxy S III / Cyanogen Droid launch screen, but never got farther. It did give a vibration as it usually does on boot.
The next day, it would fire the camera flash when I inserted the battery and would light up the indicator LED blue when the volume down was pressed. The LED was faint to light red when trying to boot, but no screen response. I pulled out the motherboard and wiped everything down with cotton balls of rubbing alcohol to remove residue from the water, but it was hard to tell if it was chlorine or just the thermal paste off the processor/heat sink contacts.
For the last 5 days it hasn't been doing anything other than the red and blue LED and the vibrations. I've tried leaving it to charge, and it gets warm behind the SIM and SD slots where the CPU and such are.
It's been in a bag of rice for 8 days and I don't see any improvements. I've considered rinsing the non display components in some distilled water or alcohol to "wash" it out per iFixit.
Any further suggestions?