Bring a large pot of water to a boil. Add 1 tablespoon of salt. Meanwhile, fill a large bowl with several cups of ice, and then fill it almost to the top with cold water. Set aside.
Add the shrimp to the pot of boiling water, stir, and allow the water to come back to a simmer. Cook the shrimp for 3 to 4 minutes, just until cooked through (opaque throughout). Immediately drain the shrimp and submerge them in the ice water bath for 10 minutes. Drain. Peel the shrimp and cut them in half cross-wise. Cover, and refrigerate until ready to use in the salad.
In a large bowl combine the olive oil, lemon juice, onion, and salt and pepper.
Add the shrimp, avocado, tomatoes, corn, jalapenos and scallions and toss to coat with the dressing. Transfer to a serving bowl. Garnish with the cilantro, if using.