Blend watermelon and lime juice in a food processer or blender until smooth. Strain through a fine sieve and discard solids. You should have just less than 1 cup of juice.
Pour over ice with chilled club soda or sparkling water. Makes about 16 oz.
Recipe by Alison's Allspice at https://www.alisonsallspice.com/watermelon-lime-refresher/