Sure, you could see both in one day. Add Naples and a Herculineum, too. Some people do all of Europe in 7 days, but they don't see anything. Pompeii is well worth savoring, unless you just want to zip in, see one or two half-fallen structures to say you've seen ruins, and get on you way.
We'd rather not risk not getting to the ship on time by doing both. Save something for next time. And the next.
However, the reason for the reply is to tell you how to get to Pompeii on your own. Take the local train, Ferrovia Circumvesuviana, from either Naples' Stazione Circumvesuviana in Corso Garibaldi OR from the Stazione Centrale (downstairs), runs every half hour for the 45 minute ride. It's cheap, full of locals, especially young people going surfing. Pompeii has a modern part on the sea side of the tracks, as well as the ruins on the mountain side. Keep in mind that there are two Pompeii stops; get off at the second, which is called "Pompei Scavi/Villa dei Misteri" stop. Then follow the road to the right of the Circumvesuviana stop to the tourist office at the bottom of the hill. There are restrooms at both entrances, as well as one inside the massive ruins. Take a couple of bottles of water, plus some rolls and fruit for a snack or picnic, and have a wonderful day. You can get a private guide (ONLY get one with an official badge from Coop Touring or GATA; fend off the unofficial guides that swoop down on you like the plague.) Or you can be more relaxed and wander at your own speed. You will find at least a hundred groups with guides, pause and listen at whatever building you're particularly interested in. BTW, you might find that many of the former private homes are locked with a chain across the gate. Ask a guide to open one or several for you; a Euro or half will get the job done, and take a closer look at how they lived. The locked ones are in the process of excavation and refurbishment, and well worth a look.
The ship will dock in the Stazione Marittima in Naples. You can just walk out of the dockyard, and catch a taxi from Via C. Colombo to the Stazione Circumvesuviana, or you can the turn to the right on the same street, walk a mile or so up to Corso Giuseppi Garibaldi, turn inland and you can find the Stazione Circumvesuviana will be up a few blocks and on your right.
Depending upon how long you stay at the ruins, there might be time to explore the old fortress directly to the left of the dockyard when you return in the afternoon. The less famous ruin, BTW, is Herculineum (or Erculano, as the Italians call it) is on the same line, about 15 minutes out of Pompeii--smaller than Pompeii, but more things are better preserved. Have a great cruise, and just don't try to do it all or you'll wear yourself out and not enjoy anything.
