Slice limes in half crosswise, juice the limes, reserving juice for another use, and turn the rinds inside out. Scrape membranes and flesh from lime halves and turn peels back to form small cups with the green rind on the outsides. Place halves in a small dish, edges touching, to keep cups standing and upright.
Stir watermelon gelatin mix with boiling water in a bowl, mixing until gelatin is completely dissolved. Let cool to room temperature and stir vodka into gelatin. Pour gelatin mixture into lime peel cups. Refrigerate until set, 2 to 4 hours.
Use a sharp knife to cut each lime cup wedges. Sprinkle wedges with black sesame seeds.
