Cut the watermelon in half lengthwise.
Cut along the inside edge, about ½ inch away from the rind, with your knife at an angle facing toward the center of the watermelon.
Cube watermelon. Make one inch cuts each direction to create a crosshatch pattern. Use a large serving spoon to scoop out the cubes.
Scrape out the rest of the watermelon, creating a smooth bowl, leaving ½ inch of red flesh remaining. This ½ inch layer helps the Jell-O bind with the rind.
In large bowl, combine the Jell-O and gelatin. Add the 4 cups of boiling water and stir until dissolved. Add the vodka and stir to combine.
Evenly pour the vodka jello mixture into each half. Refrigerate until set, about 3 hours.
Place watermelon half on a folded towel to keep from rolling while in fridge.
To serve, cut each half into 1-inch slices, then cut each slice into three wedges.