finely grated lime or
COMBINE sugar with water and salt in a small saucepan and set over medium. Cook, stirring frequently, until mixture starts to bubble and sugar is dissolved, 3 to 5 min. Remove from heat.
WHIRL half the watermelon in a food processor until no chunks remain. Pour into a 9×13-in. baking dish. Pour sugar mixture into food processor and add remaining watermelon. Whirl until no chunks remain. Pour into baking dish. Add juice and zest. Stir to mix evenly. Cover pan with plastic wrap.
FREEZE until ice crystals form at edges of dish, about 40 min. Using a fork, scrape and stir to evenly distribute ice crystals. Repeat every 40 min until frozen, about 2 ½ more hours. Scrape granita down length of pan, forming icy flakes. Scoop into bowls and serve. Granita will keep well, covered and frozen, up to 5 days.