How about drinking boiled water? Lots of people claim it's very soothing. Add a slice of lime or lemon and you have flavouring with very few carbs as well.
Stock cubes/bouillon are perhaps more savoury/satisfying, but they are a processed food and do contain carbs, so as a regular drink they're perhaps not the best idea.
Ingredients of an Oxo cube:
Wheat Flour, Salt, Yeast Extract (Yeast Extract, Salt), Maize Starch, Flavour Enhancers (Monosodium Glutamate, Disodium Guanylate), Colour (Ammonia Caramel), Flavourings, Beef Fat, Autolysed Yeast Extract, Dried Beef Bonestock, Sugar, Onion Powder, Lactic Acid
Per 100ml of Oxo as prepared by their instructions = 0.9g carbs. 100ml isn't very much, so you'd quickly add a fair few carbs to your day if you have 2 or 3 drinks of this.