So you are at this remote beach where the nearest living soul lies one kilometer away from you, and you have a watermelon that is really hot from the sun and no knife to cut it open? Just drop it in the sea for half an hour. The cool water will lower the temperature of the watermelon enough to make it enjoyable and refreshing. Then hit it hard against a rock and it will crack open in half.
Photo taken at Kaloí Liménes, Lasithi, Greece