Remove the rind and seeds from 4 pounds red or yellow watermelon. Puree in a blender until smooth, then strain through a fine-mesh sieve; discard the pulp. Prepare Orange Sorbet, omitting the orange zest and substituting 3 cups watermelon juice for the orange juice. Increase the lemon juice to 3 tablespoons. If desired, substitute (1 tablespoon Campari) for the Grand Marnier, white Curaçao, or vodka. Add more lemon juice to taste, if needed.