Yes water on the roof will help cool it.
Water on the roof for cooling.
At its lowest tech a roof misting system can be little more than a garden hose set to the mist setting and aimed across the roof.
Also called an evaporative cooling system a roof misting system lowers surface temperatures by spraying an extremely small amount of water across the roof allowing the water to cool the roof as it evaporates.
1 gallon of water consumes 8000 btu as it evaporates.
Instead of spraying water on the roof you should apply a cool roof coating to cool your roof down.
A cool roof coating can benefit a building and its occupants by decreasing energy bills through the reduction of air conditioning needs.