Isn't this going off topic?
The original question is how to make a gun that fires "water" as ammo.
"How do you propel water with the same destructive velocity of a bullet?"
Nothing about leaving no evidence of the shooter whatsoever.

Because I believe the answer lies more in the area of mechanical, thermodynamics and chemistry non of which is my area of expertise.
But here's my general idea,
1. To send any projectile over any distance and having the same destructive velocity of bullets over the air requires the projectile to have some kind of stable mass. Water is liquid form doesn't seems plausible to me so I would go with ice, or would superheated plasma and/or ionized form has any uses? Just wild guess here.

2. The idea is to propel the "ammo" to very high velocity. Common guns uses gunpowder to generate the explosive pressure required. Note that heat in this case is simply a side effect. So either we shield the "ammo" from the heat, or look for other mechanism to generate the required pressure. Compressed air, jets of water, or would repelling magnetic force, power requirement aside, all sounds plausible.

Rail gun sounds like a good choice, except pure ice is a very poor electricity conductor. So perhaps mixing it with the appropriate amount of chemical ions and it would work.