It will be less useful, more destructive, or less portable than you'd expect.
Firstly: Range. One of the big problems with water as a projectile is that it doesn't like sticking together when moving through the air. The easiest way to see this is to pour a bucket of water off the top of a tall building, you'll see that it very quickly disperses into a series of drops, rather than one cohesive lump. The reason for this is that the turbulence caused by the air rushing past the water is enough to break the surface tension of the water, except for small droplets. For your gun to have any decent range, it's going to have to push that water out fast, and as you're spraying it fast the leading edge of the stream is going to break into thousands of water droplets, rather than one cohesive stream. Why is this a problem?
Electricity doesn't jump as easily as you might think. While part of the stream might be capable of transmitting a current, most of the beam is going to be made of small droplets. From the point of view of the electrical current that's pretty much the same as a lot of air. You can get around this by pumping up the
current voltage (Thanks Michael) enough for it to bridge from one drop to the next, but at that point it will also have enough oomph to bridge down all the other drops that are spraying down towards the ground (thanks to the aforementioned turbulence) and ground far short of the target. Or possibly to wrap back around to the operator and ground through him.
We can fix this by spraying more water to keep the stream continuous, but then we reach a different problem: Volume. The further you want an uninterrupted beam of water, the more water you have to push through at a high enough velocity to hit the target. This (sadly) means we have to spray water at an awfully high rate for an awfully long time, which equates to an awful lot of water. It goes from the portable to needing a van quite quickly...
The biggest problem, however, is establishing a potential difference to actually make the electricity flow. You can't set up a circuit through the gun because you'd need two (non touching) streams of water. You can't ground the power, because that would need your gun to be at potential the entire time, which is a very good way for your operator to get zapped and not a good way to deliver a life threatening jolt of power (without the aforementioned operator zappage).
Finally: Water is a great conductor of electricity. So much so in fact that if your enemy gets wet (because you just shot him with a water gun) most of the electricity would likely route down him and hit the ground without causing any damage whatsoever. And really, who wants a dread weapon that can be defeated with a pac-a-mac?
All things considered: You'd be better off with a Taser strapped to the bottom of a super soaker. The super soaker doesn't actually do anything useful, but at least it looks cool.