Yes, if you are not doing visits with your cruise, in order to visit St Petersburg, you will have to be in possession of a tourist visa to enter Russia acquired before the departure from your cruise.
The best way to visit the city is to book a guide who, with a “tour ticket”, will meet you at the dock to commence the visit of the city.
The “tour ticket” is a detailed programme of visits done during one day or more. In general, it is prepared few days before your arrival and functions for all nationalities, e.g. French, Swiss, Canada, etc…