1) Put cut watermelon into freezer bags and store in freezer for at least 5 hours, preferably overnight. This can be done weeks in advance.
2) Put the frozen watermelon into a blender. Add all the remaining ingredients and pulse until the mixture is thoroughly blended and creamy, with flecks of coriander throughout. Pour into 4 ice cold pilsner, lager or any other tall glasses.
Garnish each glass with a coriander sprig and a lime wheel. Serve.