Place the watermelon pieces in the cocktail shaker and muddle well.
Combine the remaining liquid ingredients in the cocktail shaker. Add ice and shake until well-chilled, about 10 to 15 seconds.
Wet the rim of a chilled margarita glass with the squeezed-out lime. Pour the Tajín onto a saucer and gently dip the rim of the glass in it, coating it entirely.
Partly strain the cocktail into margarita glass, then pour without a strainer to include the muddled watermelon. Garnish with a thin slice of watermelon.