Add watermelon and mint to a pitcher and use a wooden spoon to muddle together, releasing as much juice and breaking down the fruit as much as possible.
Make the simple syrup, set aside. Juice the lime, set aside.
Add the rum, syrup, club soda, and lime juice to the pitcher. Mix well.
Rim the glasses in lime juice and salt or sugar. Fill with ice, then pour in the mojito.
Add one slice of jalapeno to each glass; muddle with your straw or spoon for best flavor.
photos by Kacy Meinecke