Reference

Free-For-All

Free-for-all is the default, unorganized water fight style. It consists of a bunch of people with weapons running around and shooting each other. The result tends to be somewhat chaotic, but you still need to have a few rules in place. These vary, so be sure to specify them before the fight begins.

Maximum Payload: what weapons are considered acceptable or not for the fight, and how many a person may carry at once. By default, there are no restrictions.
Boundaries: where you are allowed to go or not go during the fight. This always varies, so if you don't know, use common sense. Stay in a reasonable area for the number of people, don't trespass on others' property, and don't go near any place where water would be a bad idea (like inside houses).
Refill Safety: whether you can shoot someone who is refilling their weapon. By default, you cannot. It is considered bad form to stay near the refill spot so that you can shoot people just before they start refilling or just after they finish.
Alliances: whether you can form informal teams with other players. By default, assume that you can, but that no team should include more than half the total number of players unless it is officially authorized.
Time Outs: whether players can call time-outs. By default, assume that players can call a halt to the game in the case of an emergency, an injury, a need to clarify rules, or if they wish to leave -- but not for other reasons, such as "I need to escape."
Targets of Opportunity: whether you can fire on unarmed opponents, or hapless bystanders who wander into the war zone. By default, assume not.

By definition, a free-for-all does not include official or permanent teams. If your water fight has teams, it may be considered a guerilla warfare game.

Variations in Survivability

Although generally a free-for-all allows people to hang in a fight for as long as they want, some people like to cause people to "die" either permanently or temporarily in the game, removing them from play. Here are a few methods for such things:

Elimination Methods:

For permanent elimination, people are usually removed when they have been shot at and hit a designated number of times. To avoid arguments, this number is often one. Usually what qualifies as a "hit" is being struck on any part of the body or any article of clothing by a direct blast, not mist, foam, or indirect splash damage if something nearby is hit.

Rotation Methods:

Some people prefer to remove people from the game only temporarily. Players are usually removed on the same criteria as elimination -- that is, a designated number of hits -- but the reentry method varies. The most common are a certain length of time (such as a minute), an uncertain length of time (such as until clothes are dry again), or a certain location (such as a tree).

Scenarios

~ Deathmatch ~

Game Type: Free-for-all
Survivability: Rotational deaths with location-based reentry

Deathmatch can be played in a confined area by a small number of people with a large number of weapons. Designate a "safe" zone which can not be fired into or out of (you may want this to be located at the refill point). Arrange all the weapons in this area, preferably with the largest and most powerful weapons the furthest in and hardest to reach.

Everyone begins, unarmed, at least a certain distance from the safe area. When the game starts, everyone may rush to the center and arm themselves, but remember that you cannot fire into or out of the safe zone. You should make a maximum limit on the number of weapons any person can carry, usually either one or two (you may wish to divide the weapons into size classifications and allow one small and one large weapon per person). Each person may take whatever weapon(s) they like, but the first person to touch any weapon is considered to have control of that weapon. You may not take a weapon that someone else already has in their possession.

Players may shoot at each other anywhere in the play area, as long as the attacker and the target are both out of the safe zone. When someone is hit, they must place any weapons they were carrying on the ground, raise both hands above their head to show that they are hit, and return to the safe zone. Once back inside, they may arm themselves again and reenter play.

Any weapons left on the ground by players who "die" may be picked up by any live player, immediately or at a later time, but the limit to the number of weapons that can be carried still applies, so they may have to drop one of the weapons they already have. Live players may reenter the safe zone at any time to switch or refill weapons.

Armed players should not loiter in the area near the edge of the safe zone. To prevent dodging in and out, any player who fires their weapon may not enter the safe zone for 30 seconds, and is still considered a target if they are in the safe zone before that time is up, even by other players in the safe zone (who are not subject to this penalty).

The game ends after a specified amount of time, or when there is no more than one player still wishing to participate.