In a large bowl, sprinkle 3 tablespoons (3 packages) of unflavored gelatin over the cold water and allow the gelatin to absorb the water for 2 minutes. Stir in the boiling water until gelatin is fully dissolved. Stir in the sugar until it is dissolved. In a blender or food processor blend the watermelon and lime juice until it is liquefied.
Whisk the watermelon-lime mixture and the vodka into the gelatin-sugar mixture in the large bowl. Refrigerate the mixture until it thickens to the consistency of a gel. Stir to evenly distribute any watermelon flesh that may have risen to the top. Gently spoon the mixture into a 5 cup mold.
Refrigerate until firm, about 8 hours or overnight.
To unmold, fill a sink, basin, or large bowl with warm water. Dip the mold just to the rim in the warm water for about 15 seconds. Lift from water, hold upright, and shake slightly to loosen the gelatin from mold. Place a cold, moistened plate over the top of the mold and invert the plate and mold together. Carefully lift the mold, if gelatin does not release dip the mold in the warm water and try again. Garnish with a mint sprig and serve.