Watermelon seeds are low in calories and are nutrient dense. One ounce of watermelon seeds contains approximately 158 calories. One of several minerals found in watermelon seeds is magnesium. In a 4 gm serving, you’ll get 21 mg of magnesium. handful of watermelon seeds contains about 0.29 mg of iron.Watermelon seeds also provide a good source of both monounsaturated and polyunsaturated fatty acids — 0.3 and 1.1 gm, respectively.