This question has been asked and answered many many times in great detail. To view earlier replies simply type petersburg into the keyword section of the 'Find a Question' box (highlighted in blue). In here you will find many responses and I think the one to portergill of Bristol with 20 replies, should answer your question (to which I think you will find the answer to be a resounding NO!).
No, No, No not on your own. Go with a local tour company or the ship's excursions. ... Neil.