How do you make a water gun battle more fun? Bring blood into the equation. Since real blood is gross (unless you're a vampire), you can make do with Water Wars, which equips each participant with a target vest that turns bloody red when it's hit with water.
Don’t worry, it's not voodoo. Instead, the vests use a layer of hydrochromatic ink. When dry, the special ink is virtually invisible to the naked eye. Once it comes in contact with water, it reacts by turning bloody red.
Each Water Wars kit comes with two water pistols and two vests. Each vest is marked with a round target area in the front torso, covered with the hydrochromatic material. The idea is to turn the target on your opponent's body completely red in order to win.
As far as I know, the red will disappear as soon as the area dries up, so stalling your opponents should help you "recover," adding another extra dimension to water gun shootouts. Of course, if you have overactive sweat glands, you'll probably turn the thing red without ever being shot, so make sure to wear clothing that will help keep sweat away.
Suck UK has Water Wars on their products list, but doesn't have it on stock. There's no pricing either, but it sounds like something fun to look forward to.