Soak the jalapeño slices in tequila in a jar; let them sit for 15 minutes so the spice infuses the tequila. After 15 minutes, remove the jalapeño slices.
Mix the salt and chili powder on a plate. Smear the rim of the glass with a lime wedge, then dip the rim into the salt and chili powder mixture, coating the rim completely.
Add ice to each glass.
Mix the juices, simple syrup, and infused tequila in a cocktail shaker and shake vigorously.
Pour into the glass, garnish with a watermelon slice, and enjoy!