...the face with the water hose like maybe they looked at you while they were walking by your fence or something...

It's appropriate in many circumstances.

1-- It's a close friend, and lets face it, thats darn funny.

2-- If you turn around with the hose and he just happens to be there staring at you. Say it was an accident....he was being a creep.

3-- When they quite physically ask you to spray them in the face. But be careful - it may just be someone looking for a lawsuit.

only in fun and only if the other person would share in the humor

yes, if they give you the stink eye, i say spray them!