Drawing directly onto your watercolor paper can be tricky.  While it makes for a faster start as you would not have to transfer the drawing onto the watercolor paper later, using pencil directly onto your watercolor paper requires a soft touch and confidence in your drawing skills.  A light hand is needed to avoid gouging or damaging the paper surface.  The lead of the pencil may seem soft but it can be hard enough to damage paper fibers especially if it has a sharpened edge.  Pressing the pencil too hard on the paper can also cause permanent depressions.   Indentations on the paper can trap paint  passages will have a tendency to go towards the deeper indentations on the paper and you might find marks appearing where there should be none. Erasures should also be avoided or minimized because the abrasion caused by the eraser may be enough to disturb paper fibers which can also affect its ability to take in paint.  If you cannot help the erasing, use the very soft white erasers.  I favor Staedtler’s because it is the softest I have tried so far among what is available locally but there are a lot of soft erasers out there you might want to try.   White eraser is preferred because some colored erasers leave stains behind.  I also would advice getting kneaded erasers.  These pliable gray erasers are perfect for picking up loose graphite.  Don’t rub it on (it can be abrasive to the paper this way) but just sort of roll it like a log on the paper’s surface.   Knead the eraser when its surface gets dirty.