Add the watermelon, lime juice, sweetener, salt, 1 cup of the water, and jalapeño if using to a blender. Blend on high until completely smooth, about 30-60 seconds.
Pour the mixture through a fine-mesh strainer into a large pitcher, pressing the pulp with a spoon to extract as much liquid as possible. Discard the pulp.
Stir in the remaining 1 cup of cold water. Taste and adjust sweetness, lime juice, or heat to your liking.
If rimming the glasses, run a lime wedge around the rim and dip in Tajín or chili-lime salt.
Pour over ice and serve immediately. Garnish with lime wedges, fresh mint, or extra jalapeño slices if desired.
