Screen not working on water damaged iPhone and can't disable Find My iPhone to restore
You can just put your device into DFU mode, which bypasses the Find My iPhone requirement. To put the device in DFU mode: 


Connect the device to a computer running the most recent version of iTunes (can be downloaded at https://www.apple.com/itunes/download/)
Hold down the Home button and the Sleep/Wake button for 10 seconds
Once 10 seconds has elapsed then release the Sleep/Wake button
Keep holding the Home button until the device appears in iTunes


If you need more help getting your device into DFU mode look at http://www.iclarified.com/1034/how-to-put-an-iphone-into-dfu

From here you will be able to restore your device using iTunes.