On further thought, a 'stream of liquid' gun would not be that great as an assault weapon for damage on its own. Rather, I suggest two types of gun.
The 'Trident': One is a small gun that just shoots a stream of chem-water, much like the chem spray. The effect would be reducing fire stacks (2 stacks per 1 second?) and adding a chemspray buff for the usual amount of time (without the timeout). It could cause a very small amount of damage with extended use, perhaps more to the balloon than anything. Infinite ammo, no reload, short range particle. Can be used on friend or foe ships.
http://www.flickr.com/photos/jpbrady/6874805218/sizes/z/in/photostream/or
The 'Aquilon': The second gun would be a medium, air-powered device that fires a volley of liquid nitrogen or highly compressed CO2 canisters. These explode on contact, causing all sorts of havoc in a localized area (little burst or AOE). On its own, it could do medium shatter damage, moderate piercing, and instantly puts out any fires as a side effect. If it hits a chemsprayed component, however, it would do heavy shatter and piercing, as well as moderate hull damage. 8-12 ammo capacity, rapid fire rate, long reload (to pump up the launcher compressor), short range physics projectiles. No friendly fire.
On their own, neither is a frightening gun. The Aquilon would be basically a shorter range Hwacha as far as damage is concerned, and the water cannon is just a big squirt gun. Combined, however, they would be quite frightening and effective.
