In a medium skillet cook oysters in simmering, lightly salted water for 1 to 2 minutes or until edges curl. Drain well. Cut the shrimp into bite-size pieces.
In a large bowl combine oysters, shrimp, avocado, tomato, cocktail sauce, onion, parsley, lime juice, chili pepper, hot pepper sauce, and garlic. Cover; refrigerate for several hours. If desired, serve in individual lettuce-lined dishes with crackers and lemon wedges. Makes 8 servings.