When creating watercolor paintings that are rather detailed, one tends to draw in the details first with a pencil. Those who are picky about their drawings tend to spend way to much time erasing their pencil lines. Number one tip, draw less and then you'll end up erasing less often. However, if you are one who has to draw and erases either a lot or occasionally, I highly recommend using a kneaded eraser. They leave relatively little to no eraser dust behind. When it gets dirty, simply knead the eraser, twist and pull, fold it around on itself ... and voila - a clean eraser! Picture number one shows my kneaded eraser torn in half. The left piece is the dirty piece, the piece on the right has been kneaded and is now clean. You can also shape the edge to fit a rather small place if you need to erase something small or narrow.
Picture 2 shows my eraser test. I drew two horizontal lines. Then erased a spot directly below each eraser. On the top line I brushed away the eraser dust with the back of my hand. On the second line, I left the eraser dust in place. A pink eraser will sometimes leave a smudge of pink on watercolor paper. The gum eraser leaves behind a lot of eraser dust plus sometimes leaves a smudge of graphite from the pencil. I suppose a white eraser would be my second choice in a pinch. The best eraser for erasing pencil marks on watercolor paper is the kneaded eraser.
Draw lightly in pencil before painting in watercolors. If you have made your pencil drawing too dark on your watercolor paper, you can use a kneaded eraser to lift up some of the pencil line. Just blot the lines with the eraser, knead and repeat, do not rub.
One little extra note ... once you paint over or spread clear water over top of a pencil mark, it's there to stay.