What is the best thing to outline a sketch with when you're going to use watercolour afterwards?
- Sharpie thick point
- Sharpie fine point
- Regular black pen
- Black colour pencil
As you gave sharpies as option, you seem to want see the drawing when finished with the water colour.
In that case, use whatever waterproof marker you happy to draw with, as the lines will be well visible, you need to draw in such a way that the lines are as you want them to be in the end result.
Pencil lines will work as well, if less obvious, as long as the pencil is waterproof, as most are.
On the other hand, if you want to lines to be almost invisible, a very light and thin pencil line or even pencil of which the lines will dissolve in water and disappear mostly or all in the paint you use later.
So from that you see it is a design decision.
Show or hide the lines, your choice.
Because watercolor layers are transparent and thin, any hard pencil marks will most likely be visible through the paint at the end.
THE "NICE" WAY
I recommend using your pencil lightly when you are creating your initial sketch/map and erase at least partially afterward, if possible.
I also recommend you using a 9H pencil or some "H" pencil, they're light and not noticeable.
THE "CHEAP" WAY
Get a pencil, use it lightly, don't make it too noticeable.
Hopes this helps! Good luck with your painting!
Considering you didn't add additional requirements for the appearance of your intended piece, I'll take into consideration ease of use, practicality, and longevity.
Watercolour pencils contain pigments that act like watercolour when water is added, basically being dry watercolour in stick-form, so they will simply mix with your regular watercolours, and dissolve. In my experience they leave traces of lines every here and there, little specks of dry but encapsulated pigment.
Watercolour markers. Similar to the pencils, these are basically watercolours in stick-form.
Similar in appearance to graphite, charcoal was used for underdrawings by nineteenth century illustrators for their watercolour pieces.
The trick is to use as little as possible, buff it into the paper (results with different kinds of paper may vary), and going over it with single, light layers, waiting each time for the layer to dry. You can add a little Arabic gum to seal those first layers in and fixate the charcoal.
To be sure, you can always test different methods in advance: use small pieces of the paper your intended work will be created on, draw fine lines with whatever you fancy, wait until these are completely dry, and add a layer of watercolour to see how they interact. You can seal of the first layer using an artist fixative or a mix of water and Arabic Gum.
Ultimately, this is a personal and creative choice, and dependent on the intended look.