Bring a medium pot of water to a boil and poach the shrimp for 2 minutes until pink and opaque.
Immediately transfer the shrimp to an ice bath to stop the cooking process, then drain and chop into bite-sized pieces.
In a large glass bowl, whisk together the lime juice, lemon juice, orange juice, olive oil, sea salt, black pepper, and garlic powder to create the marinade.
Add the chopped shrimp to the marinade, toss to coat, and refrigerate for 15 minutes to allow the flavors to meld.
While the shrimp marinates, dice the cucumber, tomato, and red onion, and finely chop the fresh cilantro.
Remove the shrimp from the refrigerator and gently fold in the cucumber, tomato, red onion, and cilantro.
Just before serving, dice the avocado and fold it in carefully to keep the pieces intact, then serve chilled.