Presuming you have a high energy source, the issue of concern is not the conversion, but how to maintain it.
Plasma loses cohesion and converts to gas the moment it leaves a confined space, or expands beyond a certain amount. This isn't particularly useful for guns.
The three best methods I can think of are as follows (neither are viable outside of a high energy physics lab with current technology)
- Induct a self-replicating A/C current into the ball of plasma to hold it together. This will remain active long enough for the plasma to reach it's target without dissipating.
- Generate and accelerate the plasma along an EMI phase-beam (i.e. maser, or laser for current high energy physics, although EMI Phase-Beams in other bands would work.) Presuming sufficient power,the phase beam would act as a magnetic containment device providing a nice beam of plasma.
- Ionize the plasma and utilize an small ionized projectile for it to be attracted to.
As of a handwavium power source. Any number of "high energy power cells" could be used, or a micro-fusion generator.
As of a handwavium water generator. You could condense humidity out of the air. You only need a few microliters to make a deadly amount of plasma if you make the plasma hot enough. Considering parts of the gun need to be superheated, and parts need to be supercooled, you already have a handwavium grade heat transfer device. The supercooled parts can double as condensers.
If you are using a projectile for ionic plasma isolation. that tiny projectile package can contain everything: a one time use power cell, a microliter of water, and the projectile.