In a blender, puree the watermelon, raspberries, sugar, creme de cassis, and lemon juice in batches until smooth. Strain through a fine sieve into a large metal baking pan, pressing down to extract as much juice as possible.
Freeze, stirring every 30 minutes with a fork to break up the forming ice crystals, until firm but not completely frozen, 2 to 3 hours. To serve, scrape with a fork and spoon into footed coupe glasses.
Preheat a fryer to 350 degrees F. Thoroughly dry the mint leaves with paper towels. Place the mint in the fryer and fry for 15 seconds, just to crisp up. Remove and drain on a paper towel-lined plate. Garnish with the mint and serve.