Yes, there is the water gain with dbol, but there is also the drastically increased protein synthesis which builds muscle, along with the increased nitrogen retention and the anti-catabolic effects.
Besides, you can gain muscle on dbol without holding water. Many people, including me, have had success running dbol along with arimidex. Nothing but lean tissue gain, similar to anavar. So while dbol does cause water retention, it also has distinct muscle building properties that drol simply does not.