Cut the watermelon in half straight down the middle, and scoop out the flesh from both halves. Trim the base a little (be careful not to cut a hole in the bottom) so they sit upright.

Put half the flesh in a small blender (saving the rest to eat, or for a second batch of punch) with all the other ingredients. When blended, pour the cocktail back into the watermelon halves, and add four straws. Pass them around.