HELP! I just dropped my iPhone 4S into the toilet! What do I do?
Well, prevention is better than cure, but you're past that point now so it's too late to give you a lecture! This procedure will also work for the iPhone 4, as well as earlier models and iPods, but don't try to take these devices apart).
First off, you're iPhone's been dropped into the best liquid possible for a successful repair. Water, while perfectly capable of destroying delicate electronic devices is a lot better than other liquids. Beer and sodas are worse because they can cause more corrosion and the sugar makes it hard to remove the residue.
Chlorinated water, such as swimming pool water, is much worse and can severely damage the circuitry. Salt or sea water is by far the worse and can cause massive corrosion inside the handset in a short period of time.
If your handset's been damaged by anything other than water, I suggest that you seek a professional repair. My advice here relates only to water damage.
Here's what you should do:
Fish your iPhone out of the water as quickly as possible.
Power it off immediately. Running power through wet electronics can damage them beyond repair.
Dry the outside of the handset. Remove the SIM card tray and SIM card and dry them off.
If possible, remove the rear glass panel off your iPhone. Apple uses two special 5-point Pentalobe screws to secure the back, but I've managed to remove these little nasties with a small, straight-edged jeweler's screwdriver. Take care not to strip the heads of the screws.
Find a Ziploc bag and fill it with either uncooked rice or silica gel, if you have access to it, and dump the handset into the bag and seal it up. Don't try to be clever and use heat to try to speed up the drying process as you're likely to damage the handset.
Leave the handset in the bag for a day or two.
After a couple of days take out the handset. If you took it apart, then you'll need to reassemble it, and see if it powers on. If it does, you're in luck.
For people who are prone to getting their electronics wet, it might be worthwhile investing in some BHEESTIE dry out bags. They're not cheap, but they're very effective.
One problem that I've both heard about and seen personally following repairing a flooded iPhone is that the battery can be damaged and no longer hold a full charge. If you notice this then you'll need to replace the battery. Fortunately, this is both quick and easy to do -- as long as you can get past those Pentalobe screws.