The following items have the corresponding effect.

Umbrella: Put this on and it will rain around you, causing people to get soaked.
Water Bomb: Throw this at somebody to get them really soaked.
Raincoat: Put this on and you will be temporarily invincible.
Cork: Pick this up and find a gun on the floor. Point at it and use the cork. If someone picks up the gun, that gun will be plugged.
Bucket: Really soak someone by throwing a bucket of water at them. This will cause their speed and jump height to decrease.
Plastic Bottle: Use this to recover some water.
Bath Towel: Use this item to dry yourself off.
Chirpy Hammer: Use this and you will be temporarily invincible. You can also hit people with it.
Skates: Use this and your speed will increase.
Banana: Causes people to slip and glide for about five seconds.
Sticky Juice: Combine this with your squirt gun. Anyone who is hit with the juice will get their speed and jump height reduced Note: This does not recover water.
Surprise Device: Many things can happen. For example, it might get dark, you might get dried, or you might get hit in the head with a bucket, etc.