Worse, if you look at the component packaging there is a thermal profile that is to be followed to ensure that the component does not experience thermal shock from expansion and contraction. Thermal shock can disable components or cause them to be intermittent. A thermal profile looks like this:
Water will create thermal shock because of it's low boiling point and high specific heat (capacity to absorb heat), it's proabably one of the fastest ways to cool down a PCB or part. Another way to do it would be to turn a can of dust off upside-down and spray your parts down. But you don't want to do that, the parts will cool down too fast and you could break them.
In fact it's probably a good idea to slowly back away the hot air gun away from the part to let the temperature ramp down. Or turn the temperature down on the heat gun and let the part cool off a little before removing the heat.
For large parts such as BGA's a thermal profile isn't just a good idea, the part will not function correctly if the thermal profile is not followed. Because the pads on a BGA are so small, and the solder connection so small that thermally shocking the solder can introduce discontinuities in the solder connection itself. The nice hot air guns for BGA's can also follow a profile.