Carving the watermelon: Select a ripe, oblong watermelon that has a flat bottom. Carve off the top quarter of the watermelon and set aside to use for sails.
Scoop out all of the watermelon innards and set aside for making punch. Once the watermelon's empty, use a pumpkin carving set to carve notches and portholes into the sides of the watermelon to make it look like a pirate ship.
For the sails, scrape out the 1/4 shell of watermelon rind down to the white portion. Cut sails from either end of this piece and thread them through skewers. Stick the skewers into the bottom of the watermelon (being careful not to poke through the bottom) and use extra pieces of rind for stability.
Make the punch: Add 2 cups of the reserved watermelon to a blender with mint.
Puree until mint is finely chopped and watermelon is frothy.
Strain watermelon and mint mixture through a sieve.
Pour 6 cups of watermelon-mint juice into large pitcher and add lime juice and vodka.
Pour this mixture into the carved watermelon, add large ice rounds and top with prosecco.