Cook the spaghetti as usual way (around 1.8 L water will need need around 10 cc salt, taste the water so it's not too salty or too bland)

In a separate pan, put the tomato sauce and the cream and let it simmer at medium heat so that excess water will go out (reduce) for around 4 minutes. Add sugar and salt, stir.

Put the prawns in, cover and let it simmer 4 minutes, then put the scallops in, cover and let it simmer 2 minutes, turn the heat off.

By this time, the spaghetti should be "al dente". take them out and mix them with the sauce. Serve them.