Ahh the dreaded toilet fall! Make sure your device is off, and get ready for some dis-assembly. Its not as daunting as you may think, and with some patience you have a very high chance of saving your iPhone.
The best thing to do at this point is to dis-assemble your device completely. Purchase some 99.9% isopropyl alcohol, some silica gel (the little packets with the small balls that say do not eat), and start cleaning the internals. Water can corrode the internals of an iPhone to a point that it can fry the matherboard.
Once your device is dis-assembled, place the components inside of the silica gel for anywhere from 12-24 hours. Once you remove the components from the silica gel, clean all connection points, flex cables, and other areas with the 99.9% isopropyl alcohol. Make sure you don't leave behind any areas that look corroded. I like to use a small magnifying glass to thoroughly inspect the connection points, and make sure I didn't miss anything. After you clean everything, you can put your device back together, and see if you have successfully saved your device! We have a had a super high success rate with saving devices with this method.