In a medium bowl, combine the scallops, 1/2 cup of the lime juice, and 1 teaspoon salt and set aside at room temperature for exactly 1 hour, stirring occasionally.
In a separate bowl, combine the cucumbers, shallots, scallions, avocado, bell pepper, parsley, jalapeno pepper, and garlic. In a glass measuring cup, whisk together the remaining 1/4 cup lime juice, 1/4 cup olive oil, 1 teaspoon salt, 1/2 teaspoon pepper, and the Sriracha. Pour over the vegetables and combine.
When ready to serve, lift the scallops out of the lime juice with a slotted spoon (discard the liquid) and add them to the vegetable mixture. Stir well and spoon into the lettuce leaves for serving.