Step 1: Prepare your fruit. Wash and chop your chosen fruit into manageable pieces. If using watermelon, remove seeds. If using berries, simply rinse them well.
Step 2: In a high-speed blender, combine the chopped fruit, 3 cups of cold water, and your desired amount of sweetener (if using). Blend on high until the mixture is completely smooth, about 30-60 seconds. Add more water if the mixture is too thick to blend.
Step 3: Place a fine-mesh sieve over a large pitcher. Carefully pour the blended fruit mixture through the sieve, using a spoon or spatula to press against the solids to extract as much liquid as possible. Discard the pulp.
Step 4: Add 1 tablespoon of fresh lime juice to the strained fruit liquid in the pitcher. Stir well and taste. Adjust sweetness and tartness by adding more sweetener or lime juice as desired. If the fruit agua fresca is too concentrated, add an additional 1/2 to 1 cup of cold water.
Step 5: Chill the fruit agua fresca in the refrigerator for at least 30 minutes, or until thoroughly cold. This enhances the refreshing qualities of the drink.
Step 6: Serve the fruit agua fresca over ice, garnished with fresh fruit slices or mint sprigs, if desired. Enjoy immediately for the best flavor and refreshment.