actually the fruit of watermelon called the melon, just as the fruits of other plants of the Gourd family (Cucurbitaceae) - melon, pumpkin, cucumber.
Then why say that watermelon - berry?
And the fact that the melon is a type of berries! After all, what is a berry? Botanists call the berry fruit with lots of seeds and juicy flesh, covered with a thick leathery shell. Learn portrait watermelon?
This means that the melon - berry. And cucumber, and zucchini, and squash, imagine, also berries!