For the watermelon juice: Chop a watermelon into chunks, cutting off the rind and removing seeds. Whirl in a blender until pureed. A 5-pound melon should yield about 1 quart of juice.
For the tarragon salt: Muddle together dried tarragon leaves and salt. Use a mortar and pestle if you have one, rather than a food processor; the mixture comes together better.
For the simple syrup: In a small saucepan over medium heat, stir together 1 cup water and 1 1/2 cups sugar until dissolved. Cook two minutes, remove from heat and cool. You'll have a lot left over; a bar staple, it will keep for at least a month in the refrigerator.
Place the watermelon juice, tequila, orange liqueur and 2 tablespoons simple syrup into a blender. Squeeze the lime over the top of it, saving the rind to run over the edge of the glasses. Add the ice and blend until the liquid is slushy smooth.
Rub the rim of a medium glass with the lime rind, then dip it in the tarragon salt. Pour the margarita. Garnish with a lime, melon ball or something unique, like a lychee nut.
To make this drink virgin: Replace the tequila and orange liqueur with pineapple juice.