To poach the seafood, you will need a large saucepan fitted with a colander insert. In the bottom of the pan, combine the celery, carrot, onion, bay leaf, lemon juice, salt, and water. Bring to a boil.
Place the calamari in the colander and cook in the simmering water for 2 minutes. Remove from the water, transfer to a large bowl, and set aside. Place the scallops in the colander and cook until opaque, about 3 to 4 minutes. Remove from the water and transfer to the bowl with the calamari (large sea scallops may be cut in half horizontally after cooking; bay scallops may be left whole). Place the shrimp in the colander and cook until pink, about 3 minutes. Remove from the water. Peel, devein, and cut in half horizontally, adding them to the bowl with the calamari and scallops.
Remove the colander from the pan and place the octopus directly in the saucepan, adding more water if necessary to completely cover. Bring to a boil. Reduce the heat and gently simmer until tender, about 40 to 50 minutes. Transfer the octopus to a bowl and rinse under cold running water until octopus is completely cooled, about 5 minutes. Use your fingers and a small paring knife to remove the outer skin. Discard the hard mouth and head sac. Cut the tentacles and octopus on the bias into 1-inch pieces and add to the bowl with the other seafood.
To prepare the salad, toss the julienned celery, carrots, bell peppers, scallion, parsley, basil, and garlic together in a serving bowl. Add the seafood and toss. Add the lemon juice, vinegar, and olive oil and toss well. Season with the salt and generous grindings of pepper. Arrange 1 cup of lettuce on each serving plate. Top with equal portions of the seafood and vegetable mixture, and serve immediately.
Variations: 6 to 8 steamed mussels may be added to the basic recipe and tossed with the other seafood. To extend the recipe and add a new texture dimension, 1/2 cup cubed, peeled, cooked Idaho potatoes and/or 1/2 cup cooked or canned (drained and rinsed) cannelini beans may be added to the basic recipe.