Watermelon is not bad for dogs in moderation.
But if you give your dog too much watermelon then they could get diarrhea, stomach pain and bloating.
But in moderation giving your dog a piece of watermelon or a bowl of watermelon is okay and a healthy treat for them.
The watermelon is very hydrating for the dog as well.
I feed my dog a bowl of watermelon when I have watermelon in the summertime.
The dog mostly eats dog food but since he's such a good dog I give him a treat of some fruits such as watermelon, cantaloupe, bananas etc.
Just don't feed the dog grapes as they are toxic to dogs and can cause kidney failure.