According to Spirit of Health, a watermelon cleanse is when a person eats only organic, fresh, seeded watermelon for three straight days. This helps to clear out the kidneys and liver and improve their function.
Because watermelon is 92 percent water, it causes a diuretic effect. Care2 notes it also contains a large amount of the amino acid citrulline. Citrulline is converted to arginine, which removes ammonia and other toxins that are flushed out during increased urination. Lastly, watermelon is a good source of potassium. This helps counterbalance the high-salt diets people consume and decrease the load on the kidneys.