In a medium bowl, combine the shrimp and lime juice. Ensure the shrimp are fully submerged in the juice. Cover and refrigerate for about 2 hours, or until the shrimp turn pink and opaque.
After the shrimp are cooked in the lime juice, drain off some of the juice, leaving a little for flavor.
Add the diced tomatoes, cucumber, red onion, cilantro, jalapeño, salt, and black pepper to the shrimp. Gently toss to combine all ingredients.
If using, gently fold in the diced avocado just before serving.
Serve chilled with tortilla chips on the side.