I just returned from the May 3 Moscow to St Petersburg trip. In St. Pete you dock close to the green line, M3, needing a short bus ride to connect to about 2 blocks from the ship. If you skip some of the optional tours, it is easy to get to Church of the Spilled Blood, take the tour, and walk to the Russian Museum close by. Our program director was happy to let people off downtown after a morning included tour, letting us make our way back on our own. Felt very comfortable on both subway and bus, good signage in roman letters as well as cyrilic. Moscow subway signage not as good, but counting stations works, and then a 10 minute walk from subway to ship.
the schedules are pretty full in both cities, so it is hard to squeeze these excursions in, but it can be done.
In summer GCT runs optional canal trips, and it looks easy to walk on to a tour on your own, but don't know if narration in english is available.
It was a great trip.