Bring water to a boil in a large pot. Add clams, cover, and cook until shells open, about 10 minutes. Transfer clams to a large bowl, reserving cooking liquid. Discard any clams that do not open. Remove meat from shells, and return to bowl. Discard shells. Pour reserved liquid through a fine sieve lined with cheesecloth into a large bowl (you should have 2 1/2 cups). Sprinkle a few tablespoons liquid over clams to keep them moist.
Heat 1 tablespoon oil in a large pot over medium-high heat. Add pancetta, and cook, stirring occasionally, until golden brown, about 7 minutes. Remove pancetta with a slotted spoon, and drain on paper towels. Pour off excess drippings, leaving just enough to coat bottom of pot. Add remaining 1 tablespoon oil, and reduce heat to medium. Add onion, celery, garlic, and red-pepper flakes, and cook, scraping bottom of pot, until vegetables are light gold, about 7 minutes.
Raise heat to high, and stir in Marsala if desired. Add tomatoes, 1 cup reserved tomato juice, 2 1/2 cups reserved clam broth, and the potato. Reduce heat, and simmer until potato is tender, 8 to 10 minutes.
Cut large clams in half. Stir clams, pancetta, parsley, and oregano into broth, and heat until warmed through, about 30 seconds. Season with salt and pepper, and serve immediately.
Some clams are naturally salty, so taste the soup before adding salt. The soup can be made a day in advance through step 3 and refrigerated overnight. Before serving, rewarm gently over low heat. Add the clams, pancetta, and herbs, and heat until warmed through.