Combine the quartered celery, smashed garlic, half of the lemon juice, 10 cups water, and salt and pepper to taste in a large saucepan. Cover and bring to a boil, then uncover, add the carrots and cook 2 minutes. Add the shrimp and cook until opaque, about 4 minutes. With a slotted spoon, transfer the carrots and shrimp to a colander and rinse under cold water. Return the water to a boil, add the scallops and cook until opaque, 1 to 2 minutes. Remove the scallops with a slotted spoon, add to the colander and rinse under cold water.
Ladle out 1/3 cup of the poaching liquid; set aside to cool. Dice half of the avocado and combine with the sliced celery, lettuce, capers and croutons in a large bowl. Add the carrots, shrimp and scallops.
Puree the remaining avocado half in a blender with the reserved poaching liquid, the remaining lemon juice, the caper brine, chopped garlic, mayonnaise, 1/2 teaspoon salt, and pepper to taste. Toss with the salad and season with salt and pepper.