A watermelon that is rotten may appear dull or sunken, with a discoloured, dark or mouldy spot. The skin may also have a mushy or soft texture in some areas.  It may also have a strong, unpleasant odour that is often described as sour or fermented. If you notice a strong smell when you cut into the watermelon, it's a sign that it's gone bad. Another telltale sign is a slimy or mushy texture. If the flesh feels soft or disintegrates when you press on it with your fingers, it's a sign that the watermelon has started to rot.  And if all this is not enough for you to tell a watermelon is rotten and you bite into one, the taste will be sour or bitter.