I found this:

"Cannabis doesn't cause dehydration. The dry mouth effect colloquially called cotton mouth is caused by the activations of CB1/CB2 cannabinoid receptors in the sub-mandibular glands which inhibits the production of saliva."

Add mouth breathing and you get even more dry mouth.

Constipation from cannabis use is not from dehydration. It is from the effect cannabis has on the GI tract. The papers report both sides of this same problem, especially with frequent and sustained use.