Dogs will have diarrhea if they eat too much watermelon, because watermelon is a cold food. If the dog eats too much at one time or eats it frequently, it will easily cause diarrhea, so dogs should not be allowed to eat too much. It’s okay to eat more watermelon. It’s okay to give your dog a little watermelon occasionally. It can help your dog relieve heat and thirst, replenish body moisture, and supplement vitamins.