For granita, bring grapefruit juice and sugar up to a simmer and stir until sugar is dissolved. Remove from heat and cool. Purée grapefruit syrup with watermelon and strain. Stir in ¼ cup vodka and freeze. After an hour, stir granita, and then stir 2 more times before frozen.
For lime cream, whip cream to soft peaks and fold in sugar, juice and zest. Chill until ready to serve.
To serve, scrape up granita granules with a fork and spoon into dessert glasses. Top granita with a small dollop of lime cream and garnish with fresh mint.