Cut watermelon flesh away from rind. Remove seeds if needed. Roughly chop.
Blend watermelon on high until completely liquified.
Strain through a fine mesh strainer pressing pulp firmly. Discard solids.
Add sugar, lime juice, lemon juice, salt, and lime zest. Whisk until sugar dissolves.
Taste and adjust. Make slightly sweeter and more tart than ideal before freezing.
Pour into a shallow freezer-safe dish in a layer no deeper than 3cm.
Cover loosely with cling wrap. Freeze on a flat shelf.
After 45 minutes to 1 hour, scrape frozen edges toward the centre with a fork.
Return to freezer. Scrape every 30 minutes for the next 2 hours alternating scraping directions.
After 3 hours total the granita should be completely loose crystals throughout.
Scrape surface into fluffy mounds. Spoon into chilled glasses or bowls.
Garnish with mint, lime wedge, tajín if using, and flaky sea salt. Serve immediately.