It's interesting that your snake is experiencing this sloughing problem. All of my water snakes at times have this happen to them. Most recently, my big redbelly though shedding most of her skin, did so in peices with remnants still attached. This despite the fact that they all have large bathing dishes provided at all times.
I'm leaning towards thinking that the heat provided at their
"hot spot/shelter" is too intense and therefore drying the oils that separate the old to new skin. It is not the ambient room humidity that is the root cause. Most have under-tank heat pads (that are covered to protect the occupants from burning) which creates an extremely dry local enviroment.
I know I have to find a balance of "appropriate" heat without providing damp housing. Last weekend I purchased a humidifier hoping to overcome the dry problem BUT, humidity is difficult to control; with an ambient room reading of 45% when the reservoir tank is filled to 75% within hours of use, damp conditions prevail! Even at this, I suspect that they (the snakes) remain too dry (are baked dry) by the under-tank heater.
Even though the snakes seem to love their hot-spot, perhaps too much of a good thing is'nt so good. I'm attempting to create a more "gentle" warm area by spreading the heat out a bit. This is done by creating an inaccessible (to the snakes) air space in the tank just above the heater, thereby providing the same volume of heat but less intense or localized.
Here in Canada, with enviromental extremes often day to day, climate control is a challenge!
Michael