If using raw shrimp, bring a pot of salted water to a boil, add shrimp and cook for 2 minutes until pink, then immediately plunge into an ice bath to stop the cooking.
Peel, devein, and chop the chilled shrimp into bite-sized half-inch pieces and place them in a large glass mixing bowl.
Finely dice the cucumber, tomato, and red onion; mince the jalapeño after removing the seeds for a milder heat.
Add the diced vegetables and chopped fresh cilantro to the bowl with the shrimp.
In a small jar, whisk together the lime juice, chili powder, sea salt, and black pepper until well combined.
Pour the marinade over the shrimp mixture and toss gently to ensure everything is evenly coated.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld and the shrimp to absorb the citrus.
Just before serving, dice the avocado and gently fold it into the ceviche to maintain its creamy texture.
Serve chilled in a bowl or over a bed of leafy greens for a clean, protein-packed meal.