Bring a saucepan of water to a boil. Cook codfish in boiling water to reduce saltiness to your taste, 10 to 25 minutes; drain. Let codfish cool completely.
Mix avocado, tomato, and onion together in a bowl. Break cooled codfish into small pieces, discarding any bones, and add to the avocado mixture; stir gently. Drizzle olive oil over the mixture; add whine vinegar and stir gently. Season salad with black pepper; stir. Cover bowl with plastic wrap.
Cover bowl with plastic wrap. Refrigerate salad at least 45 minutes.