It would be helpful to know the thickness/density of your paper. The specifics, grades, etc. are beyond your question. However, you can look at the cover of your pad or pack, refer to your supplier if you buy singly, or do some experimenting.
For the sake of your question, I'll refer to the standard labeling of 90 lb, 140 lb and 300 lb. 90 being very thin (think card stock), 140 being heavy card stock and 300 being visibly thick, about the thickness of a slice of average supermarket pre-sliced cheese. These are very general terms. Look you what you use and prefer.
Now, with increased density comes increased opacity.
Thickness: "Grade" your paper on a scale of one to three (given the general descriptions above)
Texture: Your paper may be as smooth as silk (which could affect the issue—harder to "hide" pencil marks on smoother paper, harder to erase on heavy paper—though this isn't a hard-and-fast rule because I don't know your style (which is also a factor in being able to really help you).
Style: Watery? Dense? How do you prepare your paper (if at all?). There aren't too many rules with watercolor - except those you need for a specific style or goal.
First/always, miniaturize your problem. Use small pieces of the paper you prefer (don't get something different) and try these things out. You may come up with a better process than anything I've thought of):
Create the trace/transfer line image in deep dark Sharpie (or graphite) on tracing paper or vellum depending on which of my methods you choose, your style, materials, etc. The darker your trace, the more exact your composition will be. This image will either be backwards or not.
Transfer your outline on the BACK of your paper in favorable conditions so that you can see the outline through when you are looking at side you're going to paint on. If this works, tape the line drawing to the back carefully. You do NOT want it to slip. In fact, this can be done in sections, depending on your style and skill. Light tables can be fairly weak, a really sunny window can be better sometimes. Important: you may need a drape or turn out indoor lights to see enough detail, even with a lightbox.
Note: If you wet/tape your paper first, don't use anything water soluble to make your trace. If you do not wet/stretch first, experiment on small chips of paper to see what works best for you. Keep them for reference.
Stop using lines, at least at first. Think connect-the-dot pictures. If you can see your image through the back well enough, no pencils needed. Put the faintest "markers" on the front of your paper using extremely faint paint, let this dry. Use as little paint as possible. Let your dots, or hints dry. If you can't see them well enough, make sure you have a good light, tilt your paper, make your guiding dots a shade darker. Use the colors you'll be using. I have found that the faintest of lines can be seen (after drying) to guide me.
Draftsmen often use a type of pencil, a non-photo blue pencil. Try frisket to block paint from going where it shouldn't. Also a sharp chalk pencil (dove grey) if you must trace onto your surface is a possibility. Experiment on small pieces of the paper and paint you use (paint quality matters, too).
Projectors can be handy and time saving I guess, but good ones are pricey. Using printable watercolor paper could be worth a try if the quality has advanced at all. I DO use the computer to print out 8 X 10 sections of a large piece if it has to be an exact likeness. Whatever method works best for you, USE it!
I use heavy paper and all the methods I've described with success.
Again, Everything depends on your style and materials. If you care to be a little more specific on these, all of our questions would be better.
In closing, using most watercolor paper weights will allow you to be exacting in your art without touching a pencil to it.