1. Combine 1/2 cup water and sugar in a small saucepan. Bring to a boil. Cook, uncovered, 1 minute or until sugar dissolves. Remove from heat and stir in 2 (5-inch) mint sprigs. Cover and steep 15 minutes. Remove and discard mint. Cool mint syrup completely.
2. Place watermelon in a blender; process until smooth. Strain watermelon mixture through a sieve into a bowl; discard solids. Stir in mint syrup and lime juice.
3. Fill each of 2 glasses with ice cubes. Pour 3/4 cup watermelon mixture and 1/2 cup sparkling water over ice in each glass. Garnish with additional mint sprigs and lime slices, if desired.