If using raw shrimp, boil for 2–3 minutes until pink and opaque, then drain, chill, peel, and devein. If using pre-cooked shrimp, rinse and pat dry; cut large shrimp into bite-sized pieces.
Finely dice red onion, cucumber, and ripe avocado; chop cilantro; mince seeded jalapeño if using.
In a large bowl, whisk together clamato juice, ketchup, fresh lime juice, hot sauce, salt, and freshly ground black pepper until well combined.
Gently fold shrimp, diced red onion, cilantro, cucumber, avocado, and jalapeño into the sauce, taking care not to mash the avocado.
Cover the bowl and refrigerate for at least 30 minutes (up to 1 hour) to allow flavors to meld and the cocktail to chill.
Before serving, stir and adjust seasoning if needed. Serve in individual glasses or bowls with tortilla chips or saltine crackers and extra lime wedges.