1) Get a canteen or water bottle.
2) Assign it to a "quick bar" slot.
3) Press the assigned quick number
4) Press middle mouse wheel, or whatever you have assigned to perform the default action with the object in your hand
5) The bold adventurer takes a hearty swig
6) ...and takes it upon him/herself to shove it back at the bottom of your backpack ( where you also keep your head according to bug 3636 )