I drink assloads of water when I smoke (I usually polish off the 2nd half of my gallen of water while sitting here at night) and I still get cotton mouth, and red eyes
Cogeners for one. These are byproducts of fermentation to which most people have some degree of alergic reaction. Also, due to the chemical nature of alcohol - and not its direct biotoxicity - drinking depletes vitamin and minerals as well as blood sugar. All are major contributory causes to an alcohol hangover.
"Hangover" in relation to drugs is, of course, most often used with regard to alcohol. However, the term simply referes to the undesired or unintended "after" effects of a drug.
Regarding marijuana,THC, THC metabolites, and other chemicals consumed along with marijuana.
The issue could be simplified by asking are their physiological effects of marijuana after the high subsides? The answer is yes. I can certainly attest to this fact.
Whether a user may exhibit of percive these effects the following morning as missjanet asked is notuniversally answered.