You can forgo the grilling and just sprinkle the seasonings on raw watermelon. This recipe also works well with jicama.
Rub watermelon wedges with oil and sriracha. Heat a grill pan lightly coated with oil over high heat, or preheat a grill for direct heat. Grill watermelon until grill marks appear, 1 to 2 minutes per side. Transfer to a serving platter; sprinkle with salt, sugar, and lime zest. Serve with lime wedges.