Make your poaching liquid by filling a large pot with 8 cups water and 2 tablespoons salt. Bring water to a boil and add shrimp. Immediately take the pot off the heat. Leave shrimp in poaching liquid until just cooked through, about 2-3 minutes. Shrimp are cooked when firm and opaque throughout.
Remove shrimp from poaching liquid and spread onto a cutting board to cool. When shrimp are cool enough to handle, remove tails (if there are any) and chop into bite-size pieces.
Add onion, cucumber, peppers, cilantro, and chopped shrimp to a non-reactive bowl (such as glass, stainless steel, or ceramic). Pour in citrus juices and ½ teaspoon salt. Toss well, then press shrimp down into liquid as much as possible. Cover and refrigerate for at least 30 minutes or up to 4 hours.
Just before serving, stir in the avocado. Taste and season with additional salt, if needed. Serve with tortilla chips.