A bit vague, what do you intend to do with it? I know of a watercolor artist that uses a pen called "elegant writer", the black ink of that pen is composed of more than one dye, so when you use water on it separates in different colors.
But I think I would go the fountain pen route, you can put any ink you like in it, most fountain pen inks are water soluble, I use lamy black ink in the fountain pen I write my journal with. Also sketched some with it, and it dissolves readily in water without separation.
For linework I want to put a WC wash over, I prefer non soluble ink (for fountain pens, platina ink is very good for this purpose).