Sauté bacon in a large soup pot over medium-high heat until crispy, 3-4 minutes.
Remove the bacon from the pot and place on a plate.
Drain all but 1 Tbsp bacon grease from the pot. If there is not 1 Tbsp, then don't worrya bout draining the bacon fat.
Add diced onions and celery to the bacon grease in the pot. Sauté over medium heat until soft, 3-4 minutes.
Add potatoes. Saute 1-2 minutes. Reduce the heat to low.
Add the flour and stir, cooking until the flour is completely moistened.
Add the milk and increase the heat to medium.
Heat the soup until it is steaming, but just before it starts to boil, stirring often.
Keep the soup at this point until the potatoes are nearly tender, uncovered, 10 to 15 minutes. (Reduce the heat if the milk starts to foam.) The soup should start to thicken to a nice consistency.
Add the fish, shellfish, corn and bacon.
Continue to heat, just below boiling for an additional 5 to 10 minutes, until the fish is cooked through.
Add salt and pepper. Adjust seasoning if necessary.
Serve with the fresh parsley, crackers or good quality bread.