Combine coarsely chopped watermelon and sugar in a bowl and stand until macerated and juicy (45 minutes-1 hour). Process in a food processor until smooth, then pass through a muslin-lined sieve. Transfer to a 20cm x 30cm shallow metal tray and freeze, stirring and scraping occasionally with a fork until crystals form and granita is frozen (4-5 hours).
Meanwhile, for ginger syrup, combine sugar, ginger and 60ml water in a saucepan, stir over medium-high heat until sugar dissolves. Bring to the boil, remove from heat, stand until cool. Strain and refrigerate until completely chilled. Makes about ½ cup.
To serve, divide diced watermelon among chilled serving glasses and top with granita. Drizzle with a little ginger syrup and serve immediately.