Watermelon-Mint Margarita

Combine sugar, salt, 6 torn fresh mint sprigs, and 8 thin lime wedges in a medium bowl; stir to combine. Let stand 10 minutes. Pour boiling water over mint mixture, stirring until sugar dissolves; let stand 10 minutes. Strain mint mixture into a bowl, pressing on solids. Discard solids. Combine liquid, watermelon, tequila, and lime juice in a blender; process until smooth. Divide mixture among 8 ice-filled glasses; garnish with 1/4 cup fresh mint leaves and 8 thin lime wedges.