Place the diced watermelon and diced mango in a blender. Add the fresh lime juice, simple syrup (or agave), cold water, and salt. Blend on high until completely smooth, about 30–45 seconds. The water helps the mixture blend smoothly and keeps the puree slightly thin for mixing with soda.

Taste the puree. Adjust sweetness or acidity: add up to 1 more tablespoon simple syrup if you prefer sweeter, or up to 1 teaspoon more lime juice if you want brighter acidity. If the puree seems too thick, add 2–4 tablespoons cold water and blend briefly.

Strain (optional): For a smoother soda, pour the puree through a fine-mesh sieve into a bowl, pressing with the back of a spoon to extract as much liquid as possible. Discard solids. If you prefer a pulpy, fruit-forward soda, skip straining.

Carbonation & Finish

Chill glasses: Place 2 tall glasses in the refrigerator or fill with ice to chill while you finish preparing.

Assemble the soda: Fill each chilled glass halfway with ice (about 1 cup ice per glass). Divide the watermelon-mango puree evenly between the two glasses (about 3/4 cup puree per glass if not strained).

Top with carbonation: Slowly pour the chilled club soda or sparkling water over the puree in each glass, dividing it evenly (about 1 cup soda per glass). Pour gently to preserve the fizz and create a layered, slightly effervescent texture. Stir once gently with a long spoon to combine.

Garnish and serve: Add a mint sprig and a lime wheel or wedge to each glass. Serve immediately with a straw so drinker can stir again as needed.

Make-ahead note: The fruit puree can be made and refrigerated up to 24 hours ahead. Keep the soda chilled and carbonate only when ready to serve to preserve fizz.