Watermelon seeds are non toxic and can safely be eaten raw straight from the watermelon so you don't have to spit the watermelon seeds out if you don't want too.
If you want to just swallow the watermelon seeds when eating the watermelon then you can safely do so and the watermelon seeds can actually provide you with nutrition as well so they're healthy.
You can also bake watermelon seeds if you want to which makes them a good snack but eating them or swallowing the watermelon seeds raw is perfectly fine.