Put the tequila, triple sec, watermelon, juice of the limes, and 1 teaspoon of chile powder in a blender. Blend until smooth.
Cut the garnish lime into wedges. Run a lime around the rim of the glass to make it wet.
Combine the sea salt and remaining 2 teaspoons of chile powder in a small, flat dish. Roll the edge of the glass into the salt mixture to make a salt rim.
Fill each glass with ice. Pour the margarita into the glasses. Garnish with the lime and watermelon pieces if desired.
Notes
Make the drinks spicier by adding more chile to the watermelon or using hot sauce. This recipe has a mild kick in the drink itself with most heat on the rim.