Preheat the oven to 350°F (175°C). Spread the watermelon seeds evenly on a baking sheet and roast for 12-15 minutes, stirring occasionally, until they turn golden brown and emit a nutty aroma. Remove from oven and allow to cool completely.
Transfer the cooled seeds to a blender and pulse a few times to break them into smaller pieces. Add the watermelon flesh and salt, then blend on high speed until the mixture is smooth and slightly thickened, with visible seed specks.
Check the consistency of the smoothie; if too thick, add a small amount of water and blend briefly to combine.
Pour the smoothie into glasses, then serve immediately, optionally garnished with small watermelon cubes or extra roasted seeds. Observe for a creamy texture with tiny seed flakes distributed throughout.
Enjoy the earthy, nutty flavor and the smooth, speckled appearance of the finished beverage, showcasing the roasted seeds suspended in bright pink watermelon puree.