You can use lemon as a flavoring for fish, etc so it's fine for water. I made the mistake my first Dukan go of buying and drinking lots of sp water with a high sodium content even though I knew which brands to stay away from. Other than that, sp water is supposed to be great.
I'm not a fan of artificial sugars either but liberally use Stevia. I've got the herb and one itty bitty leaf tastes just like a sugar cube so I know the flavor is strictly natural! Quantity, who knows. Too much of anything...but i'm not making a salad, so it's truly wonderful.
Ginger slices, cucumber slices. If you're a fan of acidic flavors, apple cider vinegar splashed into water is so surprisingly delightful.