According to an updated Shooting Sports Manual for the Boy Scouts of America, water gun fights between human beings will not be tolerated, due to safety concerns and the possible image portrayed by engaging in such activity.
Taken straight from the manual, page 99: “Water guns and rubber band guns must only be used to shoot at targets, and eye protection must be worn.”
Likewise, the Guide to Safe Scouting, page 61, elaborates on the use of toy guns.
“Pointing any type of firearm or simulated firearm at any individual is unauthorized. Scout units may plan or participate in paintball, laser tag or similar events where participants shoot at targets that are neither living nor human representations.”
Bryan Wendall explains another basis of the rule in a Scouting Magazine blog post: “A Scout is kind. What part of pointing a firearm [simulated or otherwise] at someone is kind?”
Water balloons are still an authorized play thing, but must be biodegradable and filled to be no larger than a ping pong ball.
Other prohibited items include: Crossbows, bottle rockets, boomerangs, blow guns (this includes marshmallow shooters), Ninja throwing stars, spears, spear guns and potato guns.