We have just returned from a one week cruise on-board the Meraviglia we travelled in a group of 7 adults family & friends age range 46-86, 2 of our group had walking difficulties & sometimes needed the assistance of a wheelchair, we had 3 balcony cabins.
(1) The good points about our cruise, The Ship is fantastic, we you first step aboard it is quite breathtaking, it really is just like a 5 star Hotel with all new fixtures & fittings, plush carpets & seating, there is a good choice of bars & Lounges to sit & enjoy a drink.
(2) The 'Not so good' points, this ship was crammed with far too many passengers ( yes this is how MSC make money) during our cruise there was a notice placed on the customer service desk stating that the ship was full ( standard notice on most cruise lines) but in this instance i believe it was a true statement as in most parts of the ship you could not move around for people, let alone get in a elevator with-out people pushing.
There is a free shuttle bus operated by the port authority, this will take you to the entrance of the port where you can get all the private tour buses from, we used Civit Tours who were great, E29.50 for a full day excursion with a guided tour of Rome, fantastic value compared to the cost on the ship.
Be Warned the free shuttle bus is very badly managed, people we fighting to get on the bus, if you have a private tour booked from the port gates allow plenty of time to get there by the shuttle bus, you cant walk as it's about 3 or 4 km & would be difficult to find.
You can walk into the main area by foot from the ship or there are lots of open top bus tours available, very cheap duty free available at the port terminal before you get back onboard.
If you want a free bus transfer into Marseille follow the green path from the ship, it's only about a 10 minute walk to the bus stop, the buses run every 30 mins & will take you straight into the main port area of Marseille.
You can walk from the port into Genoa 10 minutes, it's a fantastic place to explore with it's ancient streets & buildings