For best flavor, head to the farmers' market for peak-season fruit. Freeze in a single layer on a parchment-lined rimmed baking sheet until solid.
- 3½ cups watermelon pieces, frozen; plus wedges for serving (optional)
- 1 ½”-thick jalapeño slice, with seeds; plus sliced jalapeño for serving (optional)
Purée watermelon, tequila, simple syrup, lime juice, ½”-thick jalapeño slice, and 3 cups ice in a blender until smooth and very thick (mixture will loosen immediately once poured). Divide among glasses and garnish with watermelon wedges and jalapeño slices, if desired.