If using raw shrimp, poach them in boiling water for 2-3 minutes until pink and opaque, then immediately plunge into an ice bath to stop the cooking.
Peel and devein the shrimp, then chop into bite-sized half-inch pieces.
Finely dice the red onion, cucumber, and roma tomato, ensuring all seeds are removed from the tomato and cucumber.
In a large glass bowl, whisk together the lime juice, lemon juice, sea salt, and black pepper.
Add the chopped shrimp, onion, cucumber, tomato, and finely minced jalapeño to the citrus mixture.
Toss gently to ensure everything is well coated, then cover and refrigerate for at least 30 minutes to allow the flavors to meld.
Just before serving, peel and dice the avocado into half-inch cubes and gently fold them into the ceviche.
Garnish with freshly chopped cilantro and serve chilled.