Rub rim of 8 chilled glasses with wedge of lime. Dip rims in coarse salt to coat. Chill until ready to serve. Working in batches, combine watermelon, fresh lime juice, Cointreau and tequila in a blender. Crush a tray of ice cubes in blender, then pour in 1 1/2 to 2 cups of margarita mixture and process until slushy. Pour into chilled glasses and garnish with mint, if desired. Serve immediately. Repeat with remaining ice and margarita mixture.