It's safe for dogs to eat the watermelon and most dogs love eating watermelon but before you give the dog watermelon you should remove the seeds from the watermelon whether black or white seeds.
The seeds in watermelon including the white watermelon seeds can cause an intestinal blockage so don't allow your dog to eat any watermelon seeds.
The watermelon rind can also cause intestinal blockage should the dog eat part of the rind.
The rind is not easily digestible by a dog like it is for some livestock.