My nephew dropped his iPhone 6 in the toilet while brushing his teeth. He tried the uncooked rice trick to dry out his phone but when he tried to turn on his phone after 24 hours, it only stayed on for a few seconds. Not sure if keeping the phone in uncooked rice longer would've helped. I opened up the phone, used a nail to remove the pentalobe screws, and used a heat gun to dry out the phone further but still wouldn't turn on. I let the phone sit disassembled for a couple days and still wouldn't turn on. I read a post online that changing a water damaged iPhone's battery usually fixes the issue. I tried to order a replacement battery on Amazon, but they couldn't ship it because of my Japan APO address. But iFixit.com did!
Replacement of the battery using the ifixit guide was pretty straightforward. I already had the phone disassembled and the battery removed when the replacement battery arrived so I only had to perform a few steps and voila...my nephew's iPhone 6 is operational once again.
Dry out the phone as much as possible. I recommend opening up the phone for it to dry thoroughly. If you don't have the proper tool to remove the pentalobe screws, there are other ways, search YouTube. I used a nail. Then order a replacement battery from ifixit.com. In addition to the replacement battery, the package includes all the tools you need to replace the battery. Just take your time and hopefully you'll fully restore your iPhone once again after changing the battery.