According to World's Healthiest Foods, here is why Watermelon is so great.
"Alongside of tomatoes, watermelon has moved up to the front of the line in recent research studies on high-lycopene foods. Lycopene is a carotenoid phytonutrient that's especially important for our cardiovascular health, and an increasing number of scientists now believe that lycopene is important for bone health as well. Among whole, fresh fruits that are commonly eaten in the U.S., watermelon now accounts for more U.S. intake of lycopene (by weight of fruit eaten) than any other fruit. Pink grapefruit and guava are two other important fruit sources of lycopene, although in the U.S., these fruits are more often consumed in the form of juice."