Bring about 2 cups water to a simmer in a small pot and keep hot; add it to the stew a ladleful at a time as needed to keep the seafood covered and the stew from thickening too much as it cooks.
Heat the olive oil in a 16-quart pot over medium-high heat until hot. Add the onion and cook, stirring occasionally, until softened but not darkened, 3 to 4 minutes. Add the garlic, sage, bay leaves, parsley and red pepper flakes, and stir until the sage crisps a bit but before the red pepper starts burning, about 1 minute. Add the wine and cook until the scent of the alcohol disappears, 3 to 4 minutes.
Add the octopus and cook, stirring, until it just begins to firm, 3 to 4 minutes. Stir in the pureed tomatoes and some salt and pepper. Bring the stew to a soft boil, reduce the heat to medium-low, and cook, stirring occasionally and adding hot water as needed, until the octopus is tender, about 30 minutes.
Remove the octopus, cut the tentacles into chunks, slice the head into 1/2-inch strips, and return to the pot. Add the fisherman¿s catch and cook, stirring occasionally, another 20 minutes, adding hot water as needed. Add the prawns, clams or mussels, cover, and cook until the prawns turn pink and all of the clams or mussels open, about 5 minutes.
Ladle into soup bowls, garnish with parsley and serve with toasted Tuscan bread.