Watermelon contains 90% of water, it is a great source of antioxidants (vitamin c and lycopene). Not only that it aids foods digestion but also helps in regulating the body ph (being alkaline) because our average diets contains a lot of saturated fats and sugars, and even processed foods tends to be more acidic.
It can also help in weight loss.

Good one @exyle...
Watermelon is one of my best fruits... But if I'm not thirsty, it's not always an easy choice to go for.