What's the best eyeliner for the water line?
A pencil eyeliner preferably in black, something that will stay on throughout the day without me having to re-do it every 5 seconds.

The water line? I'm not familiar with that term.
But in general, the best pencil I've found was one from sephora. Just the plain basic black pencil eyeliner from their brand. But the best non-pencil eyeliner I've found is mac's fluidliner.

It's basically the area under your eyes and above your lashes.

Oooh. I stopped wearing it there because I kept hearing it was bad for your eye to wear it there (I still don't know if it's true or not). So, since its been a long time, I'm not positive, but I think revlon's waterproof twist up pencil is what I used to use. I remember it working pretty well

It is not bad for your eye to wear eyeliner in the waterline. I asked my ex beauty school teacher. Well, I would recommend you buy an eyeliner from MAC, they work really well and stay on for a long time. They are a little more expensive but its worth it.