Bring 2 cups water and the sugar to a boil in a medium saucepan. Cook, stirring, until the sugar dissolves. Reduce the heat to medium and simmer 5 minutes. Set aside to cool. Stir in the lime juice.
Transfer the lime syrup to an 11- by 7-inch glass baking dish. Cover and place in the freezer. Stir every 45 minutes until frozen, about 3 hours. (The granita can be made 2 days ahead; cover and keep frozen.)
To serve, divide the watermelon among serving bowls. Top with granita and sprinkle with sea salt. Serve immediately.