I have not noticed a special preference of Tabac for water. However, the first time I used it, since I was using a soap that did not lather too well, I superloaded my brush. Basically, what happened was that when the lather seemed to be ok, there was still more soap in the brush and after awhile, it would dry out. This was fairly easy to identify when the lather dried out in my face. What I did was to add a few drops of water to the brush and did a bit of face lathering. I must say that after that I liked it as much as I do like lathering in a bowl. You only need to give it a few swirls to load the brush, especially if you have added a a bit of hot water to soften the soap before; otherwise, it is fairly wasteful. Since then, I have not had any problems. The bottom line is that it is always easier to add water than to remove it.
Have you checked these videos yet? If not, I think you would find them useful:
YouTube - How To Build And Apply Traditional Shaving Lather
YouTube - Introduction to Traditional Wetshaving, Part 2: Lathering
Al raz.