Make the watermelon salsa: Combine the watermelon, red onion, cilantro, lime juice and jalapeno in a bowl. Toss with 1 tablespoon olive oil and 1/2 teaspoon salt and set aside.
Preheat a grill to high. Sprinkle the fish on both sides with the chile powder and 1/2 teaspoon salt; drizzle both sides with the remaining 2 teaspoons olive oil. Brush the grill with olive oil, then add the fish and grill until marked and cooked through, 4 to 5 minutes per side. Transfer the fish to a plate and break into bite-size pieces.
Meanwhile, toss the lettuce with 2 tablespoons of the juices from the watermelon salsa and a pinch of salt. Warm the tortillas on the grill and fill with the fish, watermelon salsa, avocado and lettuce. Serve with lime wedges.
Per serving: Calories 437; Fat 18 g (Saturated 3 g); Cholesterol 93 mg; Sodium 636 mg; Carbohydrate 45 g; Fiber 6 g; Protein 25 g
Photograph by Antonis Achilleos
Recipe courtesy Food Network Magazine