We did the ship-organised excursion to St Petersburg three years ago when we cruised on the Constellation. The excursion was very good, though fairly expensive. Some people we know used Red October tours for this excursion and were very pleased with the arrangements - and with the cost! Hope this helps. Bon voyage!
We have ben twice to st.petersburg and done the ships tours, (with celebrity) although the trips are expensive they have been well worth every penny.
We got our own visas and done our own thing however I would reccomend the canal tour.
Ships tours - more expensive,larger numbers, large coaches, always having to wait for the slowest person, longer bathroom breaks. I used Alla - small group of 11, smaller minivan that can pull up directly infront of each museum, priority entry, move at your own speed, cheaper - $325pp for 2-days full tour.