Watermelon is perfectly fine for them. In fact, dogs love it. Please do NOT feed your dog grapes, raisins and avocados. The seeds of apples, cherries, peaches, plums and apricots are toxic to dogs.
Watermelon is not harmful for dogs but it is doubtful whether dogs will like it. There is nothing special in watermelon to feed the dogs forcefully. If it volunteerly like to eat it, it is well and good. However, no harm in trying it.
