Marijuana and hemp are very different in that way. Marijuana cultivation is often done indoors so pests and things are tightly controlled, and the plants may be grown hydroponically and have to be very productive at growing tightly bunched and sensitive flowers. For hemp, long stringy plants with lots of fiber that grow fast and are much less sensitive, and things like the cleanliness and potential for mold contamination etc doesn’t really matter much. I don’t have any specific stats or anything but I can definitely see how marijuana may use a lot more water.