Luckily, it's just water weight gain
If you are sensitive to sodium, or have problems retaining water, then drinking a lot of beverages that contain sodium can be a problem. It can't cause you to gain fat, though. I agree with Glen, regular water is usually the way to go
I personally keep a Brita pitcher in the fridge so I always have ice cold filtered water, and a sliver of lemon is nice. Some of the flavored waters would be an equal substitute. What are you drinking?