A less cruel (and more practical - I often don't have ice water on me) method for awakening someone unconscious or asleep, is to rub your knuckles in a rough way on the chestplate of the incapacitated person. This is very unpleasant, and will make the victim wake up (if not in a coma).

The reason all these methods work is that the body experiences discomfort, and seeks to change its current state, which often requires using the brain.

Note that this will often not work in a physically induced coma. However, the icy water in ear method will, as it works through the autonomous nervous system.

(thanks for the extra info, alex.tan :)