1. Pat steak dry with paper towels. Sprinkle both sides with chili powder, salt and pepper. Drizzle oil over both sides and lightly rub in with fingertips to coat. Let rest 25 minutes.
2. Heat a grill to medium high or a grill pan over medium-high heat. (If using a grill, clean and oil grates first.) Grill steak, flipping once, 8 to 10 minutes for medium or until desired degree of doneness. Transfer steak to a cutting board, lightly tent with foil and let rest 10 minutes.
3. Meanwhile, in a large bowl, combine watermelon, jicama, shallot, ginger, lime zest and lime juice.
4. Thinly slice steak across the grain. Serve with salad; garnish with cilantro and/or basil if desired.