If using raw shrimp, bring a pot of water to a boil, add the shrimp for 2 minutes until pink and opaque, then immediately plunge into an ice bath to stop the cooking.
Peel, devein, and chop the chilled shrimp into bite-sized half-inch pieces.
In a large glass bowl, combine the chopped shrimp with the lime juice, finely diced red onion, and minced jalapeño (remove seeds for less heat).
Allow the mixture to marinate in the refrigerator for at least 15 minutes to let the flavors meld.
Gently fold in the diced Roma tomato, chopped fresh cilantro, and diced avocado until well combined.
Season with sea salt and black pepper, adjusting to your preference.
Slice the cucumber into thick rounds to serve as the base for the ceviche or as dipping chips.