Not sure I understand — would the boat trip help you avoid the tempting food, or do you anticipate that a boat trip would include those three distinctly different cuisines? If you want them, I say rent a car and drive up from Pompeii through Napoli and Roma to Firenze and fly home from there. Or, you could skip Rome, since it is congested and expensive, and go to Siena in between Naples and Genoa.
No, I was saying, I wouldn’t be able to fit on a plane. :^)