Heat grill to medium-high. Soak 8 wooden skewers for 10 minutes.
Arrange the watermelon on a plate. Sprinkle with the onion, mint, and ¼ teaspoon each salt and pepper. Set aside.
Thread the beef and jalapeños onto skewers and season with ½ teaspoon each salt and pepper. Grill, turning frequently, 6 to 8 minutes for medium-rare.
Serve with the lime wedges and watermelon salad.