Bring water to a boil and season with salt. Add spaghetti noodles and stir around the pot. Boil for 7 minutes and drain while saving 1/2 cup of pasta water. Toss the noodles with olive oil to prevent sticking. Set aside.
Prepare shrimp, mussels, and calamari and set aside together in a bowl.
Heat a tbsp of olive oil in a skillet on medium heat, and insert half of the garlic and onions. Once the onions are semi-translucent, add crushed tomatoes and mix well. When the sauce comes to a boil, add salt, pepper, red pepper flakes, and basil. Reduce the heat and simmer the sauce until thickened, and set aside.
Heat cooking oil in another skillet on medium heat, and add in the rest of the garlic and onion. Cook until the onions are semi-translucent. Add shrimp, mussels, and calamari and cook while tossing them around the pan. Stir in white wine and season with salt and pepper.
Once all the ingredients are cooked through, stir in tomato sauce and mix well. Stir in spaghetti noodles. Add in pasta water as needed to lighten the consistency and cook for another 1-2 minutes. Garnish with parsley flakes and serve.