iPhone 4S water damage, battery replacement... It works!
Three years ago I sent my iPhone 4S through an entire wash cycle and it gave black screen of death, stuck on the Apple logo and would not restore on iTunes. It went into a storage box in the garage and two moves later, I discovered the phone and decided to try restoring it for my son. It would connect to iTunes but error out. It also would not stay on unless plugged into the computer. So it was in a constant state of restore mode.
I suspected the battery was bad and swapped it out with one purchased from ifixit. I used my own eyeglass tool to remove the back. The old battery ribbon was corroded and the new one just popped in easily after I cleaned corrosion off the circuit with strong alcohol and a q-tip. I did the appropriate battery priming and ran restore with iTunes and has has been completely functional for over a week now! A $16.95 battery made an iPhone come back to life!
Don't assume your water damaged iPhone is trash... Ifixit had an inexpensive solution to my problem. I now feel confident I can fix a cracked iPad screen and plan to do that soon!