I almost exclusively use fountain pens for my sketches and handwriting my sketchbooks. However, using a permanent fine liner is a more convenient choice.
There are many different brands of permant ink fineliners available which are suitable for applying watercolour over. Some brands are Mircon, Staeldter Pigment Marker, Unipin, Pitt, Copic Mutliliner.  Look for pens that are labelled permanent or pigmented ink. These days on the rare occasions that I am not using my fountain pens, I would use a Unipin 0.3 as my basic pen but also would use a finer version 0.1 for details.  For more about why use a fountain pen please refer to this post.

I have also used Pitt Brush pens a lot over the years (especially for headings) this has a firm felt brush tip and is fun to use.
There are also a huge variety of brush pens are not quite fountain pens, but have liquid ink and makes lots of wonderful expressive marks. Pictured above is the popular Pentel Pocket Brush Pen, and refer to the section “Brush Pens” in this post for some more details.

It is also fun to explore the use of watersoluble pens as well.

This section also includes markers – either alcohol based one (such as Copic Markers) or watercolour/pigment markers.