diet colas contain a lot of sodium, which makes you retain water. If you flush it out, you won't retain as much water.
There's less sodium in a Diet Coke than in a serving of Gatorade (40mg for 12Oz vs 110mg for 8Oz), the same as a vanilla Gu gel pack (varies between falvors), etc.
If the worst thing in your diet is a can of diet pepsi every day then I'd say you're doing a lot better than 98% of the population.