My girlfriend dropped hers in the toilet. The screen glitched and then it powered off. I quickly disassembled it and then dried it out in a bowl of rice on the radiator. After drying I saw the same symptoms as you. Testing the battery on my electronics bench, I found that I could charge it and use it fine on its own when just using it as a dumb battery, but when I tried to use it to power the phone, it would immediately die and drop its output voltage very low (0.7 volts I think). I think that it has some protection circuitry that the phone pays attention to. My theory is that dunking it caused that circuitry to disable the battery so that you don't try and use it. Lawsuit prevention and all that - a lithium fire in your pocket is a good way to lose the leg. After messing with the old battery, I did find that with it plugged in, I could convince the phone to boot if I held vol down and power to enter the bootloader and then selected 'Start' from there. It even charged the battery once booted. Problem was...
Superb guide. Got my battery replaced with minimal fuss with your kit and this guide. Well done guys and thanks :-)