If using raw shrimp, poach in boiling water for 2-3 minutes until pink and opaque, then immediately transfer to an ice bath to stop the cooking.
Peel, devein, and chop the chilled shrimp into bite-sized pieces and place them in a large glass mixing bowl.
Finely dice the cucumber, Roma tomato, and red onion, then add them to the bowl with the shrimp.
In a small jar, whisk together the lime juice, extra virgin olive oil, sea salt, black pepper, and garlic powder until well emulsified.
Pour the dressing over the shrimp mixture and toss gently to ensure everything is evenly coated.
Carefully fold in the diced avocado and chopped fresh cilantro, being mindful not to mash the avocado pieces.
Let the ceviche marinate in the refrigerator for at least 15 minutes to allow the flavors to meld before serving chilled.