Potassium is an electrolyte and a mineral that your body requires to function properly. It balances other minerals and electrolytes, helps control blood pressure, regulates water and plays a role in nerve function and muscle control. Plant foods are generally considered good sources of this essential mineral. Potassium is found in both bananas and watermelon, but while bananas are considered a high-potassium food, watermelon isn't.
The Colorado State University Extension website notes that foods with more than 300 milligrams of potassium per serving are considered high-potassium foods. Bananas fall into this category because one medium banana contains 422 milligrams of potassium. Doctors advise people with kidney disease to avoid eating bananas because of their high potassium content.
Low-potassium foods are those with less than 100 milligrams per serving size, according to the Colorado State University Extension website. A 1/2-cup portion of watermelon pieces contains 88 milligrams of potassium. People with kidney disease can eat low-potassium foods, but should consult with their doctors or a dietitian to regulate portions so as not to overwork the kidneys.