Combine watermelon and 1/4 cup sugar in a large bowl; toss well. Let stand 15 minutes. Place watermelon mixture in a blender, in batches if needed; blend until smooth. Pour puree through a fine-mesh strainer into a 13- x 9-inch glass or ceramic baking dish; discard solids. Place watermelon mixture in freezer for 45 minutes. Scrape with a fork, and stir; return to freezer. Repeat procedure every 30 minutes until granita is completely frozen.
Combine flowers, 1/4 cup water, and remaining 3 tablespoons sugar in a small saucepan over medium heat. Bring to a boil; simmer 3 minutes or until syrup just coats a spoon, stirring well. Strain syrup through a fine-mesh strainer into a small bowl; discard solids.
Place 1/4 cup yogurt in each of 8 chilled parfait glasses. Top each serving with 1/4 cup strawberries. Drizzle about 1 teaspoon hibiscus syrup over strawberries. Scrape and fluff granita with a fork; top each serving with 1/2 cup granita. Top servings evenly with basil. Serve immediately.