Put the watermelon, lime juice, triple sec, and tequila in a blender and blend for about two minutes until completely liquefied. Strain twice through a fine mesh strainer.
Place the watermelon juice and half the sliced jalapenos, if using, in a large pitcher. Set aside in the refrigerator.
Heat the sugar, water, and remaining jalapeno (if using). Cook for about 2 - 3 minutes. Set aside to cool. Strain the jalapeno.
Add the simple syrup to the watermelon. Chill.
Remove the jalapenos from the watermelon after 1 - 2 hours or until the margarita reaches your desired heat level. This is optional, or if you plan to serve the margaritas later in the day. You can leave them in or take them out if you plan on serving immediately.
Lightly wet the rims of your glasses, then dip in the salt and sugar.
Add ice and then serve the watermelon margaritas. Garnish with lime, mint, and sliced jalapeno.
Notes
You can skip the jalapenos if you don't like spice. Feel free to choose to use either salt or sugar for the rim - or a mixture of both. The recipe can be doubled or halved. For a frozen margarita, take freeze the watermelon and add ice cubes. Blend with the other ingredients.