There will be other ships in port that day but the timing is really tough to arrange connecting at the ports, there are 3 of them. The easiest is in the city center for smaller ships up to 800 or passengers, the official passenger terminal which is out at the tip of Vasilivski Island and the large cargo port that all the larger cruise ships have to use because of of its deeper channel and harbor.
Even if your other passengers were in the same port, in the case of the cargo ship port in the south west part of the city, the place is emmense and you might be 2.5 miles apart.
There is plenty of time to find other passengers from the same ship but bear in mind that the vast majority of cruisers do not subscribe to these message board.
If you do not find anyone, contact DenRus Ltd ( firstname.lastname@example.org
) and ask them to place you will a small group with similar interests.
Unfortunately, the longest any cruise ship passenger can stay in port is 72 hours unless they have Russian visas. Very few cruise passengers obtain individual visas. As a result of this law, most cruise ships stay in port for 2 days and a few stay 2 full nights and 3 days.
Your arrival will be one of the last ships of the year, the season ends September 17 this year.
The independent tour companies have lots of 1 day programs which are quite popular so a lot of ground can be covered in that short period. There is no city I know of that has more reason to stay a lot longer because even 2-3 days means 97% of the options were bypassed.
Have you seen the Costa offerings for excursions? I don't know if they offer any.
What are you interested in seeing, art, architecture, estates and palaces, museums? I can suggest some options if I knew your preferences.
Have a good voyage
In St Petersburg