Muddle 5 fresh raspberries, blueberries, 2 fresh blackberries, light agave nectar, mint leaves, and lime juice in a cocktail shaker. Stir in crushed ice and water. Cover with lid, and shake vigorously until thoroughly chilled (7 to 10 seconds). Pour mixture into a 16-oz. glass, and top with ginger ale. Garnish, if desired.