Joellen, I strongly agree with Allan 's reply just above. You have not indicated your budget level, but it is very expensive to "go places" from Positano. Sorrento is a less-exceptional, purpose-built, postwar resort town on top of a cliff, but well-situated for public transportation or car service daytrips. Are you going to Pompeii or Capri?
9PM is an exceptionally late "all aboard" time for a cruise, but that certainly makes for a low-stress trip to Naples for the Embarkation. Check to see if you have to check-in much earlier and have dinner aboard for lifeboat drill and orientation???
I would not range far away on June 5 anyway. Although heat and sun are huge issue at Pompeii, it makes some sense to use the free bag check at the Pompeii entrance and go to Naples from there, since you have to check out of the hotel. (IF you want to see Pompeii) Pompeii could be brutal on a June afternoon! It's best to go first thing in the morning. And the Circumvesuviana train (Naples-Pompeii Scavi-Sorrento) is INCREDIBLY crowded, elbow to elbow. Hard to get your bags on, and no large luggage racks, only bus-sized racks, and you can't get to them anyway.
Make sure you know how many vertical stair steps it is to your hotel in Positano, and how your luggage gets there, if the number is over, say, 200.
This entire area is tremendously crowded from May to September. It's a little early to find reliable ferry schedules, because daily life/Infrastructure is a little unkempt in Southern Italy. Seeing Positano was a must for me as well, but I was disappointed by the upscale shopping (same companies as any big European city) and crowds. I was happy to move on to the next town (8 hr car and driver from a Sorrento hotel, to see the Amalfi Coast.)