In a small saucepan, simmer water and sugar and rosemary until the sugar dissolves, about 5 minutes; let cool. In a cocktail shaker, crush watermelon with a muddler or wooden spoon. Fill the shaker with ice, then add gin, rosemary syrup and lemon juice; shake well. Strain into 2 flutes. Top with champagne and garnish each with a watermelon wedge.