Start with fully cooked, cooled shrimp and scallops. If starting raw, cook thoroughly until pearly or white and opaque, then cool before marinating. Marinate the cooked seafood in fresh lime juice in the refrigerator for 2 hours for flavor; lime juice does not replace cooking.
Add the imitation crab, red onion, tomatoes and cilantro; mix well. Fold in the avocado close to serving.
Serve immediately or keep refrigerated at 40°F or below until serving. Timing assumes seafood is already cooked and cooled; allow extra time if starting raw.