My skills are not that good to use a pen. As such, I cheat by using fine tape.
If I wanted to paint a white line the length of a ship, such as a Brocklebank, then I would paint the hull white; add a fine length of tape where the white line should be; then spray the hull black.

To prevent any bleed from the black, I would coat the tape with a varnish and let it dry completely before painting the black. When the black has dried, I gently remove the tape and, hopefully, a fine white line is left without any bleed from the top coat.

The same could be done for waterlines etc.

cheers,
Mike