Prepare your grill or grill pan. Slice the watermelon into pieces that will fit on your grill.
3 pounds watermelon
In a small bowl, combine 1 tablespoon of olive oil with the chile powder. Brush both sides of the watermelon slices with the mixture, then sprinkle lightly with salt.
2 tablespoons extra-virgin olive oil, 1/2 teaspoon ancho chili powder, 1/2 teaspoon Kosher salt
Grill the watermelon until nicely charred, about one minute per side.
Transfer the grilled slices to a cutting board. Remove the rind and any seeds, then coarsely chop the flesh.
In a food processor, combine the watermelon, tomato, cucumber, cilantro, serrano chile, and remaining 1 tablespoon of olive oil. Pulse until the mixture is somewhat smooth, but still a bit chunky.
2 tablespoons extra-virgin olive oil, 1 medium tomato, 3/4 cup English cucumber, 1/4 cup fresh cilantro, 1/2 medium serrano chile, 3 pounds watermelon
Transfer to a large bowl, then stir in the lime juice and red wine vinegar. Season to taste with salt.
2 tablespoons fresh lime juice, 1 teaspoon red wine vinegar
Refrigerate until thoroughly chilled, at least 4 hours.