Hi
I am sure no authority on low carbing - but I have found a few diet sodas that are sweetened with splenda - which I believe you are allowed to have when low carbing.
One is diet-rite and the other is one called waist watchers - both are sweetened with splenda - and contain no caffeine.
Both brands have a variety of flavors.
Although I'm sure water is by far the best thing to drink.
Hope this helps.
Sharon