I use a water soluble pen also. I tried all type of chalk markers, spent a lot of money, and didn't like any of them. They were always rubbing off before I quilted.
Thanks to Hula Hoop 1 for suggesting the pens...I ordered two right away.
On the side, my mom used to call the water soluble pens "spit pens." She sewed clothing and would just spit on her finger and rub out the marks! Of course she's the one who used to spit on a kleenex to rub a dirty spot off my face!