Preheat oven to 275F.
Cut bread into 1” cubes. Place cubes on a baking sheet. Spray the bread lightly with cooking spray and sprinkle with garlic powder. Toast in the oven for 20 minutes, or until dry.
In a large cast iron pot (or other soup pot) heat oil. Add onions, garlic, celery, parsley, sage, and red pepper flakes. Sauté over medium heat until onions begin to soften, 5 min.
Add tomatoes, water, vinegar, and salt, and bring to a simmer.
Chop seafood into bite sized pieces, and add to the soup. Simmer until the fish is cooked through, 10-15 min. (If using clams or mussels add in the last 5 minutes of the cooking time.)
To serve, place croutons in the bottom of each bowl and ladle warm stew over top.