Hey. I don't think a lot of people here have gotten into the vape pens, but my sister just got a really nice one, so I can share what I know. She got the e-go2 Twist. It's pretty sweet because on the end of the shaft is a dial that adjusts the voltage output. A little too harsh? Turn the dial to reduce the voltage and it's an easier smoke. The guy told her the atomizer thing (I just call it the vapor chamber), needs to be replaced every few months and that it's like $10 for her model. I assume you will be replacing it more often if you let the glycerin juice sit in the chamber for a while between smoking. You should regularly clean it with warm water.
You couldn't just fill it with water and smoke that. It's the glycerin in the liquid that creates the smoke.