By RAW, you can't do this.
By RAW, you can sort of do this.
To start with, Create Water is bound by the rules of the Conjuration school:
A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it. [emphasis mine]
In other words, you can only use it to create water that is at rest within a container up to three times the volume of the water created. You can't create a 40 gallon blob of water over a victim's head.
The spell does grant some exceptions to this:
[...] or in an area three times as large -- possibly creating a downpour or filling many small receptacles.
Note, however, that neither of these are going to have the effect you're thinking of. Being in a "downpour" is much less distracting than having a 40-gallon bucket dumped on your head.
The Concentration section lists several ways that concentration checks can be triggered. There are two that may apply here:
You must make a concentration check if you try to cast a spell in violent weather. If you are in a high wind carrying blinding rain or sleet, the DC is 5 + the level of the spell you're casting. If you are in wind-driven hail, dust, or debris, the DC is 10 + the level of the spell you're casting. In either case, you lose the spell if you fail the concentration check. If the weather is caused by a spell, use the rules as described in the spell's description.
This suggests the answer is "no." A "downpour" isn't nearly as violent as the weather described here, and the Create Water spell doesn't specify concentration checks.
Your next best hope is the more general "spell" section:
If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you're casting.
If the spell interferes with you or distracts you in some other way, the DC is the spell's saving throw DC + the level of the spell you're casting. For a spell with no saving throw, it's the DC that the spell's saving throw would have if a save were allowed (10 + spell level + caster's ability score).
So, does a sudden downpour of the type created by a cantrip "interfere with or distract" someone who is casting a spell? Eh... The rules don't say "no," but they don't exactly say "yes" either.
If you wish to support this action (and ignore the limitations above), it probably wouldn't be too overpowering to give the victim a reflex save. On a failed save, they are slightly distracted and must make concentration checks, non-magical flames are doused, they are moderately annoyed, and so on.
See also: Creatively targeting summoning spells