Place a 13x9x2-inch baking pan in the freezer to chill.
In a medium saucepan, combine water and sugar. Bring to a boil, stirring often, until sugar dissolves. Set aside to cool completely. (Refrigerate up to 1 week if making ahead.)
Cut watermelon in half lengthwise. Using a knife or spoon, remove melon flesh without cutting through the rind. Place watermelon rind in freezer. Working in batches, puree 9 cups of the watermelon in a food processor. (Reserve the rest for another use.)
Strain pureed watermelon through a fine mesh sieve, discarding solids. Transfer strained puree to a large bowl. (Puree should equal about 6 cups.) Stir in the prepared simple syrup.
Remove pan from freezer and pour in watermelon mixture. Return to freezer. Freeze, uncovered, for 2 hours. Using a fork, scrape across the surface of the granita to break into shards. Continue to freeze for 4 hours more, stirring and scraping with a fork every 45 minutes to 1 hour, or until mixture is completely frozen and scraped into shards.
To serve, spoon granita into frozen watermelon rind. Top with mini chocolate chips.