Water balloons are one thing you can get away with throwing at each other without the intent to hurt or anger! A family water balloon war is a fun way to stay cool or warm up, no matter the weather. However, it's possible to start a few fights with improperly thrown balloons, or to end the game with the wrong internal temperature. If you're worried about having safe water balloon wars, read this article and apply it to be sure everyone is okay by the end of the battle.
Use water of a temperature appropriate to the weather. If it's cold outside, and you're bundled up to play, don't be throwing around frigid water! Use a warmer water that's not going to be too hot and burn the players (or melt through the balloon). On the other hand, if it's a bright and sunny day, go ahead and fill the balloons with cold water to keep everyone cool -- after all, you'll warm up plenty when you're dodging the missiles!
Establish a below-elbow rule of throwing (excluding, for guys, "below the belt"). That way, no one will have to worry about welts on their face or neck, or getting water splashed painfully into their eyes. Set up a disqualification penalty for anyone who purposefully throws above the elbows (accidents can happen -- thus the line being so low), and encourage the players to aim for the legs or feet instead of higher up, to try and avoid accidents altogether.
Specify an area to be the arena. You can make this area of play as big or small as you'd like -- depending on space availability and how long you'd like the game to go on -- but be sure it's clear to everyone. Stepping out of bounds can disqualify people, or you can allot a certain amount of "safety time," where people can step outside and avoid being hit. You'll also want to specify the goal of the game -- is it to hit the entirety of the other team, or to capture a flag? And how to people indicate they've been hit? Make the rules clear before you start play.
Clean up all the balloon scraps afterwards to protect birds and other animals from dying. Rubber pieces can look like brightly-colored food to curious animals, and they aren't biodegradable, so they're also destructive to the environment in general.
Keep your balloons away from garage sales, or you might ruin the merchandise!