Servo (we can hack together some simple mechanism to drive our own motor)
Some gun or small water pump or air compressor to squirt water
Maybe we can get some small pump in hobbymarket… eg.: Air compressors can be found at aquarists/petmarket.
We probably can't use compressor to pump water directly (well we can try it…), but we can pump the air to bottle with water, which will make water squirt out through some outtake…