Cut the watermelon in half and scoop out the pulp. Remove any seeds from the pulp and cut the pulp into diamonds. Coat with the flour. Mix the egg whites with cornstarch and a little water into a batter. Heat the oil in a wok over high heat to about 250 degrees F (120 degrees C), or until small bubbles appear around a 1-inch (2 cm) cube of day-old bread dropped into the oil. Dip the watermelon pieces in the batter and add to the oil. Deep-fry until the coating becomes firm. Turn off the heat and continue to deep-fry the watermelon until light brown. Remove, drain well, sprinkle with the sugar, and serve.
