Does Diet Sierra Mist which has zero calories zero carbohydrates and zero sugar make you gain weight over drinking water?

It can possibly make you retain water. We do not have the beverage in question in Norway, but if it is anything similar to Pepsi, Coke etc., then, yes, I'd say it can. Some can gain weight, or have their weigh loss prevented because of some of the artificial things added in the soft drink. I can't remember the details... However, some artificial sweeteners are proven to increase your appetite which would be rather unfortunate for a person whilst dieting.

Water is always the best choice - I know, I think it sucks as well... and it doesn't mean we can't treat ourselves to one once in a while;)