Prepare your grill or grill pan.
Slice teh watermelon into pieces that will fit on your grill.
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.
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.
Transfer to a large bowl, then stir in the lime juice and red wine vinegar. Season to taste with salt.
Refrigerate until thoroughly chilled, at least 4 hours.