Cut the fish and seafood. Cut all the fish and seafood into the same-sized pieces. I like to make them about 1/2-inch thick. Place in a large glass bowl.
Add the veggies. Add the tomatoes, cilantro, red onion, lime juice, orange juice, and salt to the bowl with the seafood. Stir well to combine.
Marinate. Push seafood beneath the citrus juice, cover and refrigerate. If you'd like the fish "rare" only marinate for 10-15 minutes. If you'd like it more "well done" let it marinate for 1 hour and up to 4 hours.
Add avocado. Right before serving, stir in the avocado. Taste and add more salt or lime juice if needed.
Serve. Serve cold with tortilla chips or baked tostadas.