Ok, I don't know where to put this, so I'll try it here:
I bet you all remember this one:

My incredible masterplan is: Build such a device for next summer :)
I did some brainstorming and this is what I came up with:
1. The water should be stored in two seperate bottles of Coca-Cola, hanging upside down on my back.
2. Both caps (of the bottles) will have one hole. (Diameter: About half an centimeter)
2 small, flexible tubes will be stuck through them.
3. The ends of those tubes will be attached to my wrists.
4. You know the film spiderman? With two small pieces of plastic I wanna block the tubes, but this pieces should be moveable, so they let the water out when I move my hand like spiderman
5. The pressure will be build by using two outlets in the bottles (like the Supersoaker Waterguns), but only one...erm...lever(?).
This is the plan. I beg of you:
Think about it and help me spot mistakes
Make some suggestions
I don't know, if I will finish this, but if I do, it would be pretty awesome!