Keep your watermelon chilled in the fridge. When you are ready to make the smoothie, chop the watermelon into cubes and add to the blender.
Squeeze the limes and add the juice to the blender jug.
Blend well until smooth. Strain the liquid through a kitchen sieve to remove the rough seeds so that you are left with a very smooth drink. If you happen to use seedless watermelon you should still strain it so that your smoothie is perfectly smooth.