There are some pretty neat gadgets out there that help blow them up faster. For example, you could buy from "bunchoballoon" and fill up 100 in the time it normally takes you to fill up 5-6, but It is a little pricey. There is also a tool by "splashparty" that makes tying the balloons a snap (according to the video...which I warn you is extremely annoying! "Water balloons-water-water-water-water balloons...")
We typically just stick with the old-fashioned way and have two teams of two people filling up trash cans at two different water spigots. You may need a dolly (two wheeler) to move the trashcans around after they are filled...and yes, many of them will explode on you.
After you have enough water balloons, there are endless ways to have fun, but one of my favorites is "water balloon homerun derby." Simply line everyone up and have them take turns batting your easy, underhand pitches. Some will need some coaching at how to swing a bat, but you will be amazed how many people hit the balloon, even if they can't hit a baseball. If I was a little league baseball coach, I think we would use this game frequently because young people really seem to improve their batting average when they are rewarded with an exploding water balloon!
This is another game I highly recommend.* You'll have a blast!
*Personally, I think you should always explain to your young people the importance of dressing properly for any water games. When clothes get wet...well, I think everyone understand what I am saying. So, as you are inviting people to your water games activity, make sure you establish dress standards and make it clear to avoid any problems. We try to follow the biblical principle of having the "older women" talk to the "younger women" if they don't seem to get it.