Scrub and debeard the mussels. Put the mussels in a shallow saucepan, cover and place over moderate heat until the shells open, about 5 minutes. Remove from the heat and discard any mussels that did not open; set the mussels aside in their shells. Strain the liquid that the mussels released into the saucepan and set aside.
Preheat an oven to 350˚F.
In a heavy ovenproof pot over low heat, warm the olive oil. Add the garlic and fry gently until translucent, about 5 minutes. Add the tomatoes, wine, liquid from the mussels and the water. Cook over low heat for 30 minutes. Add the mussels and allow the flavors to blend for another 2 minutes. Season to taste with salt and pepper.
Meanwhile, toast the bread slices in the oven until golden, about 10 minutes. Place the slices in individual soup dishes. Ladle the soup over the toast, sprinkle with the parsley and serve immediately.
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