Splashing water on someone wakes them up because the water stimulates the nerve endings for the skin which wakes you up.
A sudden splash of water also sends your whole body into attention and is also a surprise which is also why it wakes you up.
Splashing cool or cold water over your face can also wake you up because the cold or cool water draws your circulation of blood upward, toward your head and temporarily renews your energy which makes you feel more awake.
The shock of cold water also causes you to breathe more deeply which also increases the amount of oxygen in the body.
This also leads to an increased heart rate, which then releases more blood throughout the body and makes you feel more energized.
The shock of cold water also invigorates your skin and makes you feel more refreshed.
The cold temperature of the water also stimulates your nervous system, which can help you feel more energized and alert.
If someone is truly unconscious and not just asleep do not attempt to wake an unconscious person by slapping or shaking them or by using cold water to rouse them.