In the only kind of terrain where you could actually ride a bike while carrying a water gun without wiping out hard and becoming the subject of ridicule (if not a reason to call 911) for your fellow players, the enemy'd see (or if not hear) you coming from farther away than you can launch a balloon at. They'd have time to prepare themselves, and maybe pick up a stick to wedge into your spokes *insert evil grin smiley that the board doesn't have but totally should, here*. Okay, we can all probably agree that doing that would be extremely unsportsmanlike. But the point remains clear. Bikes are way too clumsy for use in water wars in my opinion, though if you can come up with an effective way to use them, that's great.
Drive-by soaking is fun. I should grab one of my brothers/friends and a soaker or two, and go on a cruse through neighborhoods, ruining people's days. That'll be a summer activity though.