well if you want to fight water with water. its gunna take two people!
first have a man on the water heater. set watch alarms or have a signal to each other. then you need a bucket or large pitcher. fill half way with ice, then fill it with water. stir it to get it all freezing cold. add more ice.
now while your dad is taking a shower, walk in the bathroom and pour the ice water over his head! as soon as you do that, have the other person turn off the hot water valve. (either with alarm or if he/she is close enough just use a signal,easiest way would prolly be to use cell phones, just call the person and as soon as it rings thats the signal to turn it off. it takes about 3 -10 seconds for the cold water to reach the bathroom so practice once or twice)
did this to my step-bro.
so here it is.
step 1: man the water heater.
2: get icy water in a pitcher or bucket
3: send signal to turn off hot water.
4: pour ice cold water onto dad while hot water is still being sent into bathroom (make sure he gets about 1-2 seconds of hot water after being iced, it makes it sting more)
5: now dad is freezing cold and has no hot water... RUN LIKE HELL!