Ripe Watermelons should feel firm and heavy for their size and display a yellow spot on the rind where they sat in the field. Watermelons have a sweet, juicy nature well suited for fresh and lightly cooked preparations. Watermelons are a source of potassium to balance fluid levels within the body, vitamin A to maintain healthy organ functioning, vitamin C to strengthen the immune system, and fiber to regulate the digestive tract. The fruits also contain vitamins B6, B1, and lycopene, a nutrient that naturally gives the flesh its red or pink hue.