I'm on the Millie 5/31 cruise from Istanbul to Barcelona. We stay in port at Istanbul overnight, then leave the next afternoon. After we board the ship on the 31st, how free are you to come on and off the trip? We're planning on taking a private tour on the 31st, but I don't know if there are specific times you can get on and off in port, or if you can simply come and go as you please. I do know you have to get back on the ship at least 1 hour before takeoff.
You may come and go as you please. You'll need your Celebrity ID card (I think they still use them) to get on board, though. Again, think of it as your hotel, it only travels WITH you to different locations.
As far as the ship being a hotel that travels WITH you - be advised that it can also travel WITHOUT! you.....i.e., be aware of the time of departure when in each port. It will be clearly stated that paxs are responsible for getting themselves to the next port if they "MISS THE SHIP". (!)
While it is correct that while in port you may come and go as you please, just watch the clock - we've known folks who have, for whatever reason, simply NOT made it back to the ship in time for departure!
In fact, in many travel-related stores and catalogs you can find a flat pouch on a string that's made to go around your neck and under clothing. Great place to carry passports, etc.
To add another thought to the "getting back on time" issue, be sure to note that often they'll ask all passengers to be on board a half hour or so before the actual sailing time. This is especially important at stops using tenders. The last tender sometimes leaves a good half hour before actual sailing time to give them time to haul up the tenders and secure everything. If you miss the tender, you've missed the boat, so beware.
However, none of this should stop you from going and coming as you please while the ship is in port. Just carry your documents and keep your eye on the clock.