The concept of doing a pencil drawing to which you then add watercolor paint is a familiar one, yet somehow the thought of working with a "drawing medium" over the top of dried watercolor is regarded by some as "cheating". As if once you've started working with paint you can't go back. It's so not true! The division between drawing and painting is an artificial one; it's the artwork you create that matters.
A sharp pencil is the ideal tool for adding fine detail, for creating a crisp edge. Many people find it easier to control the direction and width of line with a pencil than a brush. Steadying your hand on a mahl stick increases the control further.
Keep the pencil tip very sharp and don't be lazy about stopping to sharpen it. Rotating it in your fingers as you use it helps maintain the point. If you truly hate sharpening, start with half-a-dozen identical pencils and swap them.
In the example here, I've worked on top of a watercolor painting (once it had dried thoroughly!) using a dark-blue graphite pencil. Specifically, indigo from Derwent's Graphitint range (Buy Direct), which has an underlying dark earthiness to it, different to a normal colored pencil. It's also water-soluble, so it was crucial to ensure the watercolor was utterly dry! As you can see, it's enabled me to crisp up the edges and introduce shadow. Notice, for instance, how it's altered the mouth, created a shadow on the earlobe and bottom of the collar, and defined the edge of the shirt.
Obviously you don't have to use a water-soluble pencil with this technique. It was what I had to hand, but also chosen with the thought that I could turn it into paint if I wanted.