Getting from Piazza di Roma to the cruise terminal involves getting onto a people mover. Ensure you have small change (euro) for the ticket machine and its quite a hike to the cruise terminal. Not easy with cases & hand luggage, especially when its really hot!
We took a private excursion which was much cheaper than the ships. We drove along the Amalfi coast, stopped in a lovely restaurant overlooking breathtaking views. We stopped at Pompei on the way back which was amazing. The only issue with a private tour, is being sure you are back at the Ship in plenty of time.