Hose

If you get one with a handle that is adjustable, you've got one of the best water weapons. The only bad thing about it is that you really can't take it over the entier battlefield. Make sure that your hose is a long one. It's also easier if you have a lighter hose, like one of those hoses you see on TV commercials.

Spray Bottle/Liquid Soap Bottle

These things are just a bit more powerful than the old, cheap water guns you can get at the Dollar Store, but have much more capacity, and longer range. Some even have adjustable nozzles. These things work great as a backup weapon.

Giant Water Balloon

Read the articles all about it here.

Bucket/Tub

Dump some water on your opponent. It's got a ton of output. But it's very heavy and impractical. Do this only if you are in a high spot and you can pour this on your opponent from up high.

Squeeze Bag

Attach a straw to a ziplock bag. Fill the bag with water. Seal the bag. Squeeze. The water should shoot about 20 feet if your strong. You've got a quick backup weapon now.

Sprinkler aka "The Water Machine Gun"

You know those sprinklers that rotate and go "TSSK! TSSK! TSSK! TSKSKSKSKSKSKSKSKSK!"? Get one of those, set it so that it don't rotate, and you've got a great sorta homemade water weapon that has great range, lots of power and infinite water.

Ketchup Bottles