Bring a large pot of lightly salted water to a boil. Add the seashells and cook until tender, about 8 minutes. Drain.
Place bacon in a large deep frying pan over medium-high heat. Cook until evenly browned, stirring occasionally. Drain off most of the fat and add the onion to the frying pan. Cook and stir until transparent. Stir in the diced tomatoes, passata and water then bring to a boil. Reduce heat to low and simmer for about 10 minutes, or until the sauce has thickened to your liking. Serve sauce over pasta.