Sorry to hear that the power button encountered water, and you can turn your Galaxy S6 Edge on and off without the power button, you can use the same instructions for Galaxy S6 as well.
'''Turn ON the Galaxy S6 without using the power button:'''
# When the Galaxy S6 Edge+ is turned off, press and hold the volume button for a few seconds.
# While holding the volume button, connect the Galaxy S6 Edge+ to a computer using a USB cable.
# Wait for your phone to boot to Download mode.
# Then press down on the volume rocker to cancel the operation.
# After the operation has canceled, the Galaxy will reboot and turn on.
# You have successfully turned on the Galaxy S6 Edge+ without using the power button.
Hope this info helps you.