Measure out 4 cups of seedless watermelon cubes and place them in the blender.
Add 1/2 cup of cleaned, dried watermelon seeds to the blender with the watermelon.
Blend on high speed for 1-2 minutes until the mixture is smooth, with the seeds finely broken down but still slightly textured.
Pour the blended mixture through a fine mesh strainer into a large bowl, pressing gently with a spatula to extract as much liquid as possible. Discard the solids.
Add 2 tablespoons of sugar to the strained liquid and stir until dissolved. Chill in the refrigerator for at least 30 minutes before serving.
Serve the watermelon agua fresca cold, garnished with a small watermelon wedge or mint leaves if desired.