Heat the grill. Preheat your grill to medium-high heat.  Make sure your grill is clean and well seasoned.  

Make the salsa. In a small bowl mix the watermelon, mango, onion, lime juice, salt, and zest.  Stir in the cilantro and refrigerate for 15 - 20 minutes while the fish cooks.  

Prepare the fish. Pat the sea bass fillets dry and brush or spray them with olive oil.  Season with salt and pepper.  

Grill the fish. Place fish on the grill, skin side down, diagonally across the grates.  Close the lid and turn the grill down to medium.  Let the fish cook for 3-4 minutes.  Using two spatulas (or a spatula and a fork) carefully lift the fish and make sure the skin is crispy.  The fish should lift from the grill easily.  If it's still sticking a bit, let it cook for an additional 30 seconds.  Carefully flip the fish over, again placing on a diagonal across the grates.  Let cook for 4 - 7 minutes more, until the temperature reads 140°F on an instant-read thermometer.  Once the fish reaches the desired temperature, remove it from the grill.  

Serve. Top with the watermelon mango salsa and serve immediately.