So we made some frantic phone calls. The wonderful Manuela at the Porto Turistico found us a berth for six months - actually at slightly less than Nautilus Marina. We motored back out of the river, round the corner and into this spiffing harbour. It has been built inside two breakwaters, and a lot of care has been taken to try and minimise swell. This matters a lot, as it has a poor reputation for swell, and we shall see how well the curved walls and large spending beach actually work. (Right now, it's very calm!)
It can't be as bad as our first winter aboard, on a pontoon right by the seawall in Brighton. Nor even Toulon, where you are constantly rocked by ferry wash.
It is a bit soulless, but it has all facilities, other liveaboards (even English-speaking ones!), excellent security, and easy public transport links into Rome. Our plan is to get Roaring Girl set up for the winter months (stow the kayak, get the bicycles out), and then start exploring the Eternal City.