Get out those water shooters and have at it!
There are quite a few ways to do target practice.
Dixie Cups
Set up dixie cups in a pyramid and then try to knock them down. You could probably do this with Solo Cups, too, if you have more powerful water guns. You could compete with another person. Each of you could have a pyramid stack and you try to knock the other person's down.
Cans or Bottles
Put up some empty aluminum cans or empty plastic bottles up and try to knock them down!
Hanging Plates
Design some paper plate targets and then string them up and practice shooting them. Color on the styrofoam ones and the colors will run off to see if you hit it. The paper ones will be wet, so you could set up your own rules for that. You could also color the liquid, if you dare, and then you know which one you've hit.
You could also play 500 with the plates, by writing the numbers on them and see how far up you can go!
Running Targets
If you have SD6 people around, you can have them put on paper plate targets and run around. Almost like laser tag, but people could take turns with the water pistols.
You can always purchase target practice toys, too. There are several out there, similar to what you would play at a carnival. Craftier people than I have also posted how-tos on making your own carnival style target.
Essentially, the sky is the limit for how you want to play. I'm not a fan of drawing on my windows, but if you have that washable paint or chalk that people use on their car windows for celebrations, you could draw targets on windows. Have your friend or sibling or someone make a face behind the target and tah-dah! Passive aggressive emotional relief.
Don't forget, though, you want to set distance parameters. You can start with a set distance and then make it closer or farther away. How accurate is your water shooter?