For my pen and ink watercolors, I always use a Faber Castell's Pitt Artist Pen. It is an india ink pen which is also waterproof.
Waterproof is important when drawing with ink and then painting in watercolor over the top. If your ink is not waterproof when dry it will bleed and look messy.
The Pitt Pens can be purchased as a set or purchased individually. They also come in different sized pen nibs:
S - Super Fine
F - Fine
M - Medium
B - Brush Tip