How can I make a sketch under a watercolour painting and have it not be seen?
Watercolor is, by its very nature, a transparent medium and so it becomes really quite difficult to manage this. However, there are a couple of things you can do to help:


Draw very lightly and then use a kneaded eraser to lift the lines that you have left. The remaining lines will be much more faint than had you just drawn as light as you can.
Draw on normal drawing paper and then use a lightbox to show the drawing through to your watercolor paper. This way you don't even draw on your final painting.
Erase the area you're about to paint. So, do it first, rather than after the fact. If you keep it small enough, you should be able to remember. It's pretty similar to working with kneaded erasers, but you're not making the line fainter, you're removing it.
Go loose! Don't do an initial drawing at all, but use a reference and simply loosely paint it. That's one of the really neat things about this medium, it lends itself to a free style that's hard to do with other paints.


Watercolor pencils won't really work for your purpose in this situation. It's likely that the colors will be slightly different from your paints, so getting an exact match will extremely difficult, especially if you're blending your own colors. Marks from the pencils are hard to blend in a wash situation. However, you just go all watercolor pencil, then you can control your destiny as it were. I'm a fan of the medium.

Finally, line and wash with the right subjects look really great, so don't necessarily feel like you're making a sacrifice to use a liner pen. If you want it to look better, look at being a little less precise with the lines, break them up a bit, and maybe skip some areas. In other words, use the lines to create variance in the image to your advantage in those conditions.