Add the watermelon cubes to a blender and blend until completely smooth, then strain through a fine-mesh sieve into a pitcher to remove pulp. You should have about 2 cups of watermelon juice.

Add the sliced jalapeño to the watermelon juice and let it infuse for 20-30 minutes in the refrigerator, tasting occasionally until it reaches your desired spice level, then remove the jalapeño slices.

Mix together equal parts coarse salt and tajín on a small plate. Run a lime wedge around the rim of each glass, then dip the rims into the salt mixture to coat.

In the pitcher with the watermelon juice, add the tequila, lime juice, orange liqueur, and agave nectar. Stir well to combine.

Fill glasses with ice, pour the margarita mixture over the ice, and garnish with lime wedges and watermelon triangles before serving.