Watermelon juice is no trouble to make at all. A small watermelon should produce about 1 liter of juice, which is what you need for 4-6 servings. If you have a juicer, just cut up the watermelon into chunks, removing any seeds, and juice away. Otherwise, purée the watermelon in a blender, then pass the purée through a sieve placed over a bowl to eliminate the pulp.