For the Minty Basil Syrup:
- Combine he watermelon and lime juice in a food processor and pulse until just combined.
- Pour the watermelon mixture through a strainer over a 9x13 inch freezer safe baking dish.
- Transfer the dish to the freezer and let the mixture chill for about 30 minutes.
- Use a fork to break up the watermelon and then return to the freezer for another half hour. Repeat the process 3-4 more times or until the granita is completely frozen.
- Meanwhile, make the minty basil syrup by combining the mint, basil, water, and honey in a small saucepan. Bring to a low simmer and cook for about 10 minutes, stirring until the honey has dissolved.
- Remove from the heat and let cool completely. Strain into a glass jar.
- To assemble the mocktail, scoop the granita into 4 glasses. Drizzle with 2 or 3 tablespoons of the syrup, then top with the chilled soda water. Garnish with additional basil and mint leaves. Serve with a small spoon or straw.