Combine tequila with sliced jalapeno in a jar. Refrigerate for an hour or so.
In a blender combine watermelon chunks, lime juice and simple syrup. Blend until liquefied.
In a shaker of ice, add 2 ounces jalapeno infused tequila, 1 ounce Grand Marnier, 1 cup watermelon juice. Shake until well chilled.
Spread 2-3 Tablespoons salt on plate. Rub lime wedge on rim of glass, then dip into salt. Pour chilled margarita into ice filled glass. Garnish with watermelon wedge, lime & jalapeno slice. Enjoy!