You bought a watermelon for a cookout. You cut it up. Everyone ate it, somehow the same amount is still there after everyone leaves. What to do? Give it to the dog?

You can, but first remove the seeds and rind. 

Watermelon rind is thin and hard, which makes it difficult for most dogs to eat safely. Additionally eating the rind may result in gastrointestinal distress causing vomiting or diarrhea.  

You are fine to give your dog the actual flesh of watermelon but make sure to remove the seeds. Seeds can cause internal blockages. If you suspect your pup has an internal blockage immediately contact a vet. 

As with all fruit, sugar content is a major deterrent of offering watermelon frequently. Fruit for dogs should be a treat.