The night before, chop up your watermelon into cubes and freeze. I try to freeze the watermelon for 24 hours.
Take the frozen watermelon, lemon juice, lemon zest, rum and sugar and blend until well combined.
If the mixture isn't slushy enough, you can add in about 1/2 cup of ice till the drink reaches the consistency you are after.
Serve in glass and top with basil and whipped cream if you'd like.