Hello mooneydiva, and thank you.
The most recent information I have is that Princess is charging
$15 for half a day in the Sanctuary. The article (from Cruise Travel magazine) said it was so successful they needed a way to limit access. But there are plenty of quiet sheltered areas so there is no need to pay extra for this.
The Grand Mediterranean cruise on Emerald Princess would be just a magnificent choice; a new ship with just enough cruises logged to get the kinks out (we were on Grand Princess' second cruise and a lot of the crew seemed as awestruck and unsure as we were); and a really marvelous itinerary. I have no words to describe the thrill of sailing into Venice and Istanbul.
I see the other posters agree: late September is best. My choice would be Sept. 26; the crowds would be gone and the weather should have moderated. I see the itinerary is reverse (Barcelona to Venice) but I suggest that is better--you end on such a high note (not that there is anything wrong with Barcelona).
Two more suggestions, then I'll shut up!
Do yourselves a favor and book a balcony cabin or mini-suite; there are so many beautiful ports on this trip; and nothing compares to the thrill of watching from your own private balcony. Second, if you sail Venice to Barcelona, book a port (left side) cabin; if you sail Barcelona to Venice book a starboard (right side) cabin; then you will be on the correct side to see St. Mark's square as you pass--a thrill you will never forget.
Have a wonderful trip, and let us know what you decide.