Combine all the ingredients in a blender and process to milkshake consistency. Taste and add more sugar as needed.
Notes
Freeze the watermelon: Frozen chunks make it thick and frosty without ice diluting the flavour.Adjust the sweetness: Ripe watermelon may need less sugar — taste as you go.Make it dairy-free: Use your favourite plant milk.Cube ahead: Freeze watermelon cubes when melon's in season for shakes all summer.