1Bring a large pot of salted water to the boil. Add the shrimp and cook until firm and opaque, 1 to 2 minutes. Drain and run the shrimp under cold water to stop the cooking. Drain on paper towels and set aside.
2Cut the shrimp in half lengthwise and place in a medium nonreactive (glass, ceramic, or stainless steel) bowl. Add the lime and lemon juices, cover the bowl, and refrigerate for 30 minutes.
3Add the onion and serrano chile to the bowl, cover, and refrigerate for an additional 30 minutes.
4Just before serving, stir in the cilantro, cucumber, and avocado.