Instead of cutting a hole in the watermelon, we use syringes for injecting the vodka. We put a bit of tape over the tiny holes and rotate the watermelon several times as it chills throughout the day. Failure to rotate the melon means that the alcohol all settles in one place. We like a uniform balance of the vodka in the melon. Be careful! It does sneak up on you...and should be kept out of reach of children.
OK, I didn't make this today, but when I saw this recipe it brought back fond memories of parties back in college. This was, as I remember, quite tasty! :)