Preheat grill. In a small bowl, combine 1 Tbsp olive oil and chili powder. Brush on both sides of watermelon slices; sprinkle with salt. Grill watermelon 3-5 minutes per side or until grill marks form. Transfer to a cutting board. When cool enough to handle, remove rind and seeds; coarsely chop. In a large bowl, combine watermelon, tomatoes, cucumber, cilantro, onion, jalapeño and remaining 1 Tbsp olive oil. Stir in juice. Refrigerate 1 hour to let flavors meld. Just before serving, season with salt and pepper.