1. Preheat grill to high. Slice watermelon in half. Place flesh side down on a cutting board. Cutting crosswise, slice each watermelon half into 3/4-inch-thick slices. Using a pastry brush, brush slices lightly all over with olive oil. Grill for 3 minutes per side, or until black grill marks appear. Cool slightly before cutting into cubes.
2. For the dressing, combine all ingredients in a small bowl and whisk. Set aside.
3. For the salad, combine cooled cubed watermelon, cucumbers, red onion, Serrano chili, pepitas, cilantro and mint in a large bowl. Add dressing and toss gently until well combined. Add crumbled queso fresco cheese and serve either at room temperature or slightly chilled.