First of all - whatever works for you, stick with it.
Diet pop does not make you gain weight. I get so tired of hearing that. Most of the time, I see people say that when they are not being honest with themselves about what they are eating. Calories in vs. Calories out... diet pop has NO CALORIES - Physiologically impossible to gain fat from drinking diet pop.
Retaining water is not the same as gaining weight and the terms shouldn't be used interchangably. Even if the sodium in the diet pop causes water retention, over time, the body adjusts to a higher sodium intake and balances water and electrolytes. This negates any initial water retention.
Caffeine can cause insulin release which may account for cravings - however, if you CHOOSE to eat because you have cravings, any weight gained has nothing to do with diet pop, it has to do with you.
I have quit drinking regular pop on 2 different occasions in the last 7 years. Each time, I switched to diet pop. I lost about 10lbs in a week and half each time.
Now, I gained 100lbs while drinking diet pop and eating EVERYTHING I could get my hands on; diet pop had nothing to do with it.
So, I have lost 78lbs since June 7th. Diet pop is my last vice and I refuse to give it up. My daily fluid intake usually consists of 3 liters of diet pop. It has neither slowed my loss nor caused me to gain.
If drinking diet pop isn't any good for you, then don't drink it. But don't come here, without any unbiased, reliable true research and give people the idea that it will cause weight gain. Meg's post was right about experimenting to see what is right for you.