Yes you can feed your dog some watermelon and it will benefit the dogs health.
Just avoid overfeeding the dog watermelon because if the dog eats too much watermelon they could get an upset stomach or get diarrhea as a result of the watermelon if they eat too much of it.
But it's perfectly okay and safe to feed your dog some watermelon but just don't give them too much at a time.
I have a dog that loves to eat watermelon when I do and I cut up some watermelon in pieces and feed the watermelon to him while I eat mine and he loves it.