Place 4 cups of water in a large saucepan and bring to a boil. Add the shrimp and cook for 2 minutes. Drain and place in ice water bath to cool. Strain when cool. Peel and devein the cooked shrimp and cut into ¼- to ½-inch pieces and place in a bowl.
In a small bowl mix 2 cups of cold water with a teaspoon of salt. Add the onions and soak for 10 minutes. Strain and rinse thoroughly. This will reduce some of the strong flavor of the onion. Mix in the remaining ingredients, except the avocado, and refrigerate for 45 minutes to an hour. Just before serving, cut the avocado into bite-size pieces and fold into the ceviche. Serve with crackers of your choice. (According to Ana, in Guatemala this ceviche is served with saltine crackers.)