If using raw shrimp, poach in boiling water for 2-3 minutes until pink, then immediately plunge into 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.
Dice the cucumber, Roma tomato, and red onion into small, uniform pieces to ensure a perfect balance in every spoonful.
Finely mince the jalapeño and chop the fresh cilantro, adding them to the bowl with the shrimp and vegetables.
Pour the fresh lime juice over the mixture and season with sea salt and black pepper, tossing gently to combine all the flavors.
Cover the bowl and let it marinate in the refrigerator for at least 20 minutes to allow the shrimp to absorb the citrus.
Just before serving, dice the avocado and fold it gently into the ceviche to maintain its creamy texture without mashing.