A friend of mine drank distilled water, and boiled all his foods in distilled water, while getting ready for a show. Fucking guy came in under 3% and looked like he didn't have skin, just raw, striaghted muscle showing!! It's a little extreme, but he looked great!!
It does get your sodium awful low. As for coming in flat, I think if you get some carbs in the night before, your msucles will suck it up, drawing more water away from your skin, and allowing you to come in full.
Of course, there are thousnads of opinions as to what to do that last week. It really comes down to what do you feel works for you.