When people are trying to lose weight, everyone tells them to drink water. Drink lots of water. There are good reasons for this. Water helps your body processes including metabolizing fat, hydrating tissues, and carrying nutrients to where they're needed. It's calorie-free, and doesn't cost anything as long as you aren't buying bottled water.
The problem, and everyone knows this, is that water is boring to drink. In fact, if it's not ice cold, it can even make you feel queasy or sick if you're drinking it and don't really want to, but are forcing it down to fill the gnawing pit in your belly. Filling up on water is great in theory, but works poorly in practice.
For people with an eating disorder, who are particularly consumed with the effect of things on their taste buds, recommending they drink water is bordering on the inane. You're already avoiding a load of sensory pleasure from the food you aren't eating and adding a limitation on the things you drink is only making things harder, particularly when there are so many calorie-free or low calorie possibilities for beverages.
The reason most people don't drink diet sodas or calorie-free bottled drinks like teas, coffees or, flavored waters is that they're told its bad for them. It's true. Food that contains chemicals or artificial sweeteners are bad for you, and caffeine affects insulin levels and can have the potential for diet-sabotaging effects as well. That being said, failing at your diet because you are not addressing one of your core issues as a person with an eating disorder is likely going to have even worse consequences than drinking things which aren't good for you. Is it better to fail and stay greatly overweight because you went the water route and ended up eating out of boredom or to drink those chemicals and reduce the chances of overeating?
In my case, I drink a fair amount of water everyday. In fact, it is the first thing I turn to about two or three hours after breakfast when I start to get hungry. I'll usually drink about 16 oz. of ice water at that point. Later in the day, I often drink about another 16 oz. between lunch and dinner, and I drink about another 8 oz. later at night around bed-time. Between those, I'm usually going for a diet soda (1 or 2 per day) or hot or ice tea with sweetener and a splash or two of low fat milk. I love Diet Coke. I know it's not good for me, but it's a guilty pleasure that is very low on guilt. It may be a "crutch" or a stand-in for the sensory stimulation of food, but I'll use it because it is a small part in the larger picture.
I think it's important to know where you're weak and to deal with those weaknesses in the least destructive way rather than to attempt to be "perfect". It's a bit like exercising when you're out of shape. If you have to stop often and rest because you lack the strength or stamina, you can choose to not try anymore because you can't do it properly or you can just stop and rest until you get strong enough to carry on without the rest stops. If I need some diet soda or calorie-free drinks to help me eat less, then I'm going to have them.