MAC Prep and Prime Fix+

[CoolToned]

Does this really work extend your makeup, or will water in an atomizer do?

I wouldn't say Fix+ would work to 'extend' makeup - I tend to use it when heavy face makeup is looking a bit cakey, 'melts' it down a bit so I don't look like I'm wearing a mask.
Water also won't do it - it'll just mess with the makeup.
If you're looking for something to make makeup last longer, try Urban Decays all nighter spray.