Prepare gas or charcoal grill for cooking. If using charcoal, open vents on bottom of grill, then light charcoal. Charcoal fire is medium-hot when you can hold your hand 5 inches above rack for 3 to 4 seconds. If using a gas grill, preheat burners on high, covered, 10 minutes, then reduce heat to moderately high. Toss together tomatoes, watermelon, onion, chile, cilantro, lime juice, and salt. (Do not make salsa more than 1 hour ahead or it will become watery.) Pat fish dry, then brush with oil and season with salt. Grill fish, starting with skin sides down, on lightly oiled grill rack, turning over once, until just cooked through, 8 to 9 minutes total. Serve fish topped with salsa.