I have travelled to St Petersburg with
DCL on two cruises both for two days, and been through passport control there eight times. Each one the transit through immigration was no more then two minutes. We did book private tours with Anastasia direct who handled our immigration tickets so no visa was required and we avoided going off on DCL tours where you nay encounter some delays if say fifty off people get off for a coach at a time, maybe two or three coaches.
DCL has NEVER stopped any scheduled disembarkation in St Petersburg, there is always some who say they may cancel, well I have been there in times of tension like Crimea, and they didn't have an issue then.
If you want to spend ( quote from someone's trip report this year) eleven hours looking at the inside of a bus, book a tour with DCL.
If you want a fantastic time contact Anastasia, tell them Andrew said their the best, and either Katherine or the CEO Anastasia will look after you personally. Anastasia herself met me going out and coming back each day, our guide was Russia's best, and we were spoilt. We were let off early, DCL tend to tell you a disembarkation time but arrive a bit earlier to get their tours off, but whilst we were early the Anastasia team were waiting for us. If you book with them, ask them any passport worries and they will sort it all out for you.
It's a fantastic place, you can either see all the sights or spend eleven hours looking at the inside of a bus. If you want navigators and photos and links see my trip report or blog, To Russia with Mickey Bars posted live on this years 12 night cruise.
If going to St Petersburg the must sees are.
Peterhof gardens. After 10 am.
Hermitage, DCL goes in, in 55s through the busy front entrance with long queues on whisper system. Anastasia goes in a back entrance VIP tickets just your group. No queues, no waiting.
Faberge Museum.
Catherine's palace, Anastasia avoided the crowds for us, not always possible.
St Isaac's cathedral .
We flew over St Petersburg by helicopter, video link in the blog.