What do I use to outline a sketch when I want to apply watercolour afterwards?
As you gave sharpies as option, you seem to want see the drawing when finished with the water colour.
In that case, use whatever waterproof marker you happy to draw with, as the lines will be well visible, you need to draw in such a way that the lines are as you want them to be in the end result.
Pencil lines will work as well, if less obvious, as long as the pencil is waterproof, as most are.
On the other hand, if you want to lines to be almost invisible, a very light and thin pencil line or even pencil of which the lines will dissolve in water and disappear mostly or all in the paint you use later.
So from that you see it is a design decision.
Show or hide the lines, your choice.