A lot of mixed media artists use watercolour graphite pencils in their work. There are lots of different brands out there. By far, I would say my favourite one to use is the Stabilo Marks All 8046 pencil. It is the darkest and strongest, graphite-wise.
Here is a little video I did showing a comparison between a lot of different brands that are available on the market.
Here is a list of the pencils tried in the video: