Syringe Mechanism

A mechanism replicating the use of a Syringe may be used.

[Video Link: http://www.youtube.com/watch?v=1CbkirLZjpo ] .
A motor would be added to the ‘Plunger’ and ‘Barrel’ junction, the suction of water by the syringe can be controlled by the direction of motion of the motor.
While spraying the water on the objects of interest, the reverse direction of the motor would cause the water to come out.

For sprinkling water, simply fix one end of a syringe to a pipe, the other end being fixed at the end of an arm on the robot. To achieve this without hindering the motion of the bot, you have to ensure that the pipe used is long enough.  To draw water from the reservoir, put the end attached to the bot in the reservoir and pull on the hand-held syringe. Point the arm to the position where water is to be sprayed and push on the syringe to sprinkle the water.
Here are a few links you can peruse to understand exactly how to make use of the syringe mechanism :
1.       See exactly how the connections are made :  http://www.youtube.com/watch?v=jZtL4yCK7RE

2.       This video demonstrates a clasping mechanism using syringes :


Scoop Mechanism

This is a simple mechanism, where you gather the water into the ‘Scoop’ . The scoop is controlled by a rotatable motorised shaft by which the scooped water can be turned upside down and the water can be dropped onto the objects of interest.

Suction Pump Mechanism

This mechanism is slightly harder to implement. It involves a vacuum suction pump which pumps the water into a container and then sprays it using a fan system.
[Video Link: https://www.youtube.com/watch?v=_BAnnTLpros ]

Sponge Compress And Relax

This mechanism is pretty much cost effective and easy to implement. You need a chunk of sponge and a basic Gripper Contraption. You could make this by using the parts of a pair of wire strippers. Of course, a simpler option would be to keep one of the fingers fixed (i.e, unattached to a motor) and only use one finger for movement. You could use the rack and pinion arrangement to do this.
Video link: http://www.youtube.com/watch?v=u6t4G2l1CC8&feature=player_embedded ]

In this video, just fix the sponge to one of the surfaces of the gripper and let alone the other side. Just dip the Setup in the water source (The sponge will of course absorb Water), then go near the object of interest; and use the gripper contraption mechanism to squeeze the water out of the Sponge.