Keep in mind when shopping for seltzer water that brands with the USDA Organic seal are more narrowly defined than those that say "natural" or make health claims. When a beverage is certified organic, the flavorings it contains are not allowed to use synthetic solvents or artificial preservatives, come from food treated with pesticide, or come from GMO products. In contrast, the federal guidelines for "natural flavors" allow for all sorts of non-natural additives to be used.
One of my favorite seltzer brands is the Kroger Simple Truth. The USDA Organic label is right up front, the product is affordable and there are a lot of interesting flavors. My favorite is Mango Grapefruit, it is a unique and refreshing blend of tropical citrus. They also offer flavors like strawberry watermelon, cucumber melon, and pineapple that are great for summer.