One in the sim slot, one near the bottom right corner of the battery where the logic board is and the 3rd on top near the ear piece.
Image
There is also one inside the headphone/earphone jack on iPhone 5/5C/5S
So there are Total 4 water sensors on the iPhone 5/5C/5S:
- Headphone/earphone jack
- SIM tray
- near the earpiece on top
- Bottom right corner of battery.
Note: If only 1 sensor is tripped, most likely they will help him out, oddly we had one iPhone 5 that had the above the earpiece sensor PINK color [the Genius told me] when we got it replaced due to faulty battery [this was before they started doing the battery replacements in the store] however I can't say OP's experience is going to be like mine, given Apple's kindness he may be in luck