The bits about watermelon being native to Africa is correct, especially near the Kalahari desert. However, the reason for the stereotype is because watermelons, as the name suggest have a high water content, more so than other fruits in fact. They are also really high in nutrients and vitamins, but I don't know that they would have been aware of that part. Anyway, Slaves working in the fields were at risk for dehydration since it was intense labor and it's hot in the south. Nobody was passing out water bottles at the time. So they would eat slices of watermelon to hydrate and refresh themselves from working in the fields. That's the where the stereotype comes from.