I have been doing a version of ink and watercolor at least since high school. In 2001, I began to use it more frequently and made a habit of taking small handmade watercolor paper with me on travel workshops using a roller ball pen with water-soluble ink. I liked the way the ink moved and ran as I worked, creating passages within the piece. That led to a chapter in "Work Small, Learn Big," a book on sketching in pen and watercolor published by International Artist Magazine in 2003.
Subsequently, I wanted to work in a larger format, but the small pens were not appropriate for that. I finally found a watersoluble ink brush with a cartridge in the Daniel Smith catalog...a brush pen used for sumei. It took awhile to get the courage to spend over $60 for the Kaimei brush pen, but when I did and used it for the first time, I was hooked. Finding the brush pens is now easy...D. Smith still carries them and typing "oriental brush pen" in a search engine leads to many more sources.
The addition of ink to watercolor is especially helpful when working in a sketch book with limited time. If you have a good line drawing, you can add minimal watercolor paint to have a good representation of the scene. And by holding back a bit on the brush pen, the line can be less powerful in a smaller format.