Tying a water balloon is infinitely less fun than throwing a water ballon. Turns out slippery wet latex doesn't make for good grip! Enter Tie Not. It's an impossibly crafty nozzle that practically does the tying for you. Watch.
It's been around for a while now, but if you're getting serious about water fights, equipping your crew with Tie Nots is the easiest way to stack the chips on your side. You can attach it to the water hose for a quick twirl and tie or use it independently as a tying assistant. So efficient! $US9 on Amazon. [Tie-Not via Geek Alerts]