It serves as a modified water gun that can shoot Acid. The bright colored casing of this harmless weapon is due to the fact that it has been shielded with a ceramic coating, making it the perfect weapon for carrying acid loads.
The Super Soaker was a water gun created by Larami before the Great War. After the Great War, it was modified and shielded with a ceramic coating for using a canister of acid as ammunition with an ammo capacity of x10 canisters. It uses air to force the acid of five canisters out of the nozzle and into the air by using a separate firing chamber system. The acid is pumped from the canister into the firing chamber, compressing the air in the chamber. This exerts a force on the acid, thus providing the power to push the acid through the nozzle when the trigger is pulled.
The water gun is the rarest weapon in the game, since only one exists and it can be found only in a special encounter. It uses the equally unusual canister of acid as ammunition, only findable in Great Bend, Newton, and Buena Vista missions. It uses 6 AP, and allows only two shots before being reloaded. Despite this, the water gun remains a correct weapon and as acid is armor-piercing, it is sometimes useful against robots.
Emphasis on "sometimes": the water gun's acid does do penetration damage, but the raw damage count is never more than 25 points and it has a high AC cost. It really isn't very effective against heavily armed humanoid robots.