1. In a large pot, bring to a boil 4 quarts of water, salted with 2 Tbsp salt. Add the shrimp and cook for 1 minute to 2 minutes max, depending on size of shrimp. (Over-cooking the shrimp will turn it rubbery.) Remove shrimp with a slotted spoon and place into a bowl of ice water to stop the cooking.
2. Drain the shrimp. Cut each piece of shrimp in half, or into inch-long pieces. Place shrimp in a glass or ceramic bowl. Mix in the lime and lemon juice. Cover and refrigerate for a half hour.
3. Mix in the chopped red onion and serrano chile. Refrigerate an additional half hour.
4. Right before serving, add the cilantro, cucumber, and avocado.