Place tequila and about 4 slices of jalapeno in a glass measuring cup and place in the refrigerator for 1 to 4 hours depending on how spicy your jalapenos are and how spicy you want the tequila to be. Take a taste after 1 hour and decide whether it is spicy enough. If so, remove jalapenos.
Place watermelon (you may need to do it in batches) in a blender and blend until liquefied.
Pour watermelon through a fine-meshed strainer, pressing on it with the back of a spoon to get all the liquid through. Measure out 3 cups of watermelon juice.
In the empty blender, combine the 3 cups watermelon juice, tequila, lime juice, Triple Sec, and simple syrup. Add ice to achieve desired texture of your frozen drink. Blend well.
Place salt with a little chili powder mixed in on a plate. Coat rims of 4 glasses with lime juice and dip in salt mixture.
Pour blended cocktail into glasses. Garnish with lime slices and jalapeno slices if desired.
**To make the simple syrup** Combine equal parts sugar in water in a small saucepan and heat over medium heat, stirring frequently until sugar is dissolved. Remove from heat and cool. Can be stored in refrigerator for future use.