Prepare the shrimp by peeling and deveining them. If using large shrimp, cut them into smaller pieces.
In a mixing bowl, combine the shrimp and lime juice, making sure the shrimp are fully submerged.
Cover the bowl with plastic wrap and refrigerate for 15-20 minutes, until the shrimp turn pink and opaque.
While the shrimp are marinating, dice the red onion, cucumber, and tomatoes, and chop the cilantro and jalapeño (if using).
Once the shrimp are ready, drain off some of the lime juice, leaving a couple of tablespoons.
Add the diced red onion, cucumber, tomatoes, cilantro, and jalapeño to the shrimp. Mix gently to combine.
Season with salt and black pepper to taste and adjust according to your preference.
Serve immediately in a chilled bowl or plate, garnished with additional cilantro if desired.