Heat the olive oil in a pan and fry the onion and garlic for 1 minute (do not brown). Add the chopped tomatoes along with the purée, capers, water and sugar, bring to a simmer.
Add the seafood along with the Basil, cover and cook for 3 minutes, until just cooked through. Season generously to taste.
Add the spaghetti to the sauce and combine until coated. Serve immediately.