Bring a medium pot of water to a boil, add the shrimp for 2 minutes until just pink, then immediately transfer to an ice bath to stop the cooking.
Peel, devein, and chop the cooled shrimp into half-inch pieces and place them in a non-reactive glass bowl.
Pour the fresh lime juice over the shrimp, ensuring they are fully submerged, and refrigerate for 20 minutes.
While the shrimp marinates, finely dice the cucumber, tomato, red onion, and jalapeño.
Drain about half of the lime juice from the bowl, then stir in the diced vegetables and chopped cilantro.
Gently fold in the diced avocado, chili powder, sea salt, and black pepper until well combined.
Serve immediately in chilled bowls or over large lettuce leaves for a clean, high-protein meal.