As far as fruits go, watermelon is one of the lowest in calories — only 46 calories per cup (154 grams). That's lower than even low-sugar fruits such as berries (2).
One cup (154 grams) of watermelon has many other nutrients as well, including these vitamins and minerals:
Vitamin C: 21% of the Reference Daily Intake (RDI)
Vitamin A: 18% of the RDI
Potassium: 5% of the RDI
Magnesium: 4% of the RDI
Vitamins B1, B5 and B6: 3% of the RDI
Watermelon is also high in carotenoids, including beta-carotene and lycopene. Plus, it has citrulline, an important amino acid.
Arun Karthe