The water most likely comes from a replicator that is inside the holodeck. Replicators actually make the molecules and everything, so once they walk out, it won't be dematerialized along with the other hologram effects. It is the same thing as if they eat in the holodeck, it doesn't leave their body when they leave!
Kitface 19:29, June 29, 2011 (UTC): Can you imagine what it would be like to have the food in your stomach dematerialised when you leave the holodeck? Sick-inducing indeed.
In ST:VOY "The Killing Game", I could've sworn B'Elanna said something about the holodeck projecting something simulating the weight gain that would be caused by the presence of a fetus in her uterus. ProfessorTrek 06:32, August 8, 2011 (UTC)