In a medium saucepan bring water and 1 ½ teaspoons of salt to a boil. Turn off the heat and add the shrimp. Allow to cook until opaque, about 2 to 3 minutes. Immediately transfer to a colander and rinse with cold water until cool, about 1 to 2 minutes.
Chop the cooked shrimp into ½-inch pieces. Place in a large non-reactive bowl.
To the bowl of shrimp add tomatoes, cucumber, onion, cilantro, serrano pepper, lime zest, lime juice, lemon juice, and 1/2 teaspoon salt. Stir to combine. Cover and refrigerate for 30 minutes, stirring halfway through.
Right before serving stir in the diced avocado. Taste and season with more salt as desired.
Serve shrimp ceviche with chips or on tostadas.
Recipe Yield: 5 cups
Serving Size: 1/2 cup
Storing: Place in an airtight container for up to 2 days before serving.