In a large pot of boiling water, cook corn until tender, about 4 minutes. Cut off tip of each cob. Stand corn in a wide shallow bowl; using a sharp knife, slice downward to release kernels. To bowl, add tomatoes, avocados, onion, parsley, 2 tablespoons oil, lemon juice, 1 teaspoon salt, and teaspoon pepper; toss to combine.
Heat grill to medium-high. Clean and oil hot grates. Rub fish with 1 tablespoon oil; sprinkle with Creole seasoning and season with salt and pepper. Grill fish, skin side down, until skin is crisp, 4 to 5 minutes (if fish does not release easily, cook a bit longer). Using a thin spatula, flip fish and cook until slightly pink in center, about 4 minutes more, depending on thickness of fillet. Serve salmon with corn relish.
Be sure the grill is hot, and oil the grates well before cooking the fish -- this will prevent it from sticking.