Chop the watermelon into cubes and place with the orange juice, orange rind and ginger in a food processor. Process for 1–2 minutes until smooth.
Pour the mixture into a freezer box and freeze for 1½ hours or until half-frozen. Take the mixture out of the freezer and whisk again in a food processor. Return to the container. Whisk at least twice more during the freezing time. There should be plenty of air whipped into the sorbet or it will be too icy and hard. Cover and freeze completely.