Dropped phone in water, but it won't stay on off the charger

Heather, you may just run into a few more errors and problems down the road. If you do not clean your phone, you may end up with corrosion damage and failure later on. I suggest that you no longer try to turn it on or try to charge it. Disassemble it using this guide and properly clean it. Use this guide to clean it. It was written for a 3G but all the points are still valid for your phone as well. After that replace the battery, reassemble your phone and re-evaluate for any problems. To definitely answer your question, you "could" use that battery from the other phone, but I recommend a new battery. Hope this helps, good luck.