I too hate plain water, anytime of year! I drink loads of sugar free squashes. My favourite is Tesco Cherries and Berries double concentrate. It contains less than 1g of carb per glassful. I avoid squashes with saccharine in as those strangely seem to raise my blood sugars. The only way to see if you can tolerate sweeteners is to test
Lots and lots and lots and lots and LOTS of herb teas. And red bush/rooibus tea. Chai teas. Herbal 'cosy' concoctions like cardamom and cinnamon and ginger for the cold season/winter. Massive amounts so it doesn't get boring. Dilmah has come out with some truly glorious concoctions. An amazing one is chocolate and turmeric and ginger and something else I can't remember. That replaces my prediagnosis Horlicks/hot chocolate/milo/chicory and barley root drinks before bed. Delicious substitute.
Green tea with mint (50/50), with a couple of sweeteners in my case. Coca-cola Zero with the freshly squeezed juice of one lemon, with one sweetener. The highly concentrated zero +/- carb fruit juices that others have mentioned, e.g Robinsons and Tesco's own. Half and half white coffee from an espresso machine using almost-zero carb almond 'milk' 0.3g carb / 100ml, again with a couple of sweeteners.