In a medium saucepan, combine the lime zest, water, and sugar. Bring to a boil over medium heat, and cook, stirring, until the sugar dissolves. Remove from the heat. Cool to room temperature and strain out the zest. (Can be made in advance; keep in a covered container in the refrigerator.)
Chill a margarita glass in the freezer for 30 minutes. Place the coarse salt in a shallow dish or saucer. Wet the rim of the glass with a lime wedge and dip the glass into the salt, coating the top edge.
In a cocktail shaker, combine 2 tablespoons of the cooled lime syrup with the watermelon puree, lime juice, tequila, orange-flavored liqueur, and ice. Shake until frothy and well chilled, at least 1 minute. Strain into the prepared glass and garnish with a lime wedge.
*Cook's Note: To make the watermelon puree, remove seeds from fresh watermelon and cut into large chunks. Place in a blender and process until smooth and well pureed.