At 40 degrees, your body definitely wants something more interesting than plain water. Homemade flavored waters are the way to go: squeeze lemon, orange, or mint into a pitcher, throw in some ginger if you're into it, and let it sit in the fridge; plus you know exactly what's in it.
If you want something ready to grab on the go, unsweetened iced teas (or ones sweetened with stevia/erythritol) from decent brands work pretty well, though some do have that weird aftertaste you mentioned, so try before you commit. What I'd avoid is hunting for "new" options just because they exist; in heat like that, even plain ice water with a splash of fresh juice will get you through the day 🧊