I have heard that coconut bark can really dry out the air in an enclosure. Therefore if you use it, you may need suplimental humidity.
You want to give the snake some options...a big water dish to drink out of, soak in or deficate in. Maybe the snake does not like to soak in water, even if it need the moisture so give it another option like a humid hide.
At the very least...put the water dish on the warm side and cover most of the screen top with a towel or something to keep the humidity in.
It sound like you have a good set up though...your snake should be happy & healthy.