Struck? As in, failed the Reflex save or took any damage at all?
I agree with Demiurge that the damage is insignificant at their CR. Maybe 3d8?
I could see stunning, but nauseated actually strikes me as the most appropriate. Imagine the stench of swamp water mixed with stomach juices!
Does this look OK?
Swamp Spit (Ex): Bosk giants can spit swamp water combined with their gastric juices at enemies. This is a ranged touch attack with a range of 30 feet that deals 3d8 points of acid damage. Additionally, a struck victim must succeed on a DC X Fortitude save or becmoe nauseated for 1d6 rounds. The save DC is Constitution-based.
A bosk giant can store enough swamp water to spit twice. After that, it may spend a full-round action to swallow enough swamp water to spit again. A swamp giant may refill its stomach after only a single use, but may never store more than two uses worth of swamp water.
Considering that the original text has the spit in a line 12 by 6, why don't we make it a breath weapon instead? A line, I would imagine.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.