I just wrapped up a visit to St. Petersburg utilizing the 72 hour visit free rule, and I thought I'd give a brief recap of my experience as I couldn't find a ton of recent experiences here aside from Andrew's in 2016 (which was still super helpful).
I booked my trip back via the Moby/St. Peter Line website in early March for travel dates May 15-18/19. It was roundtrip from Helsinki on the Princess Anastasia. I had seen conflicting information about whether you had to book a St. Petersburg hotel via their site or could use something like Booking.com or book directly with a hotel. The website didn't appear to require you book a hotel with the ferry, but I ultimately decided I didn't want to risk getting turned away at the port and booked directly with Moby. The hotels are more expensive through the site than they are if you book them directly or go to Booking.com, which was irritating but again, not overpriced enough to make it worth the risk of getting turned away. In hindsight, I'm still not convinced it's actually required to book through Moby/SPL but the rep I spoke with after booking claimed "rules" had changed and it was required, though I was trying to cancel my hotel at the time (more on that in a minute) so she may have just been trying to save the sale.
Following my online booking, I received a confirmation for the ferry (I booked the most inexpensive room I could as I was traveling alone and didn't need much for a night). My room was 75.50 euro Helsinki > St. Petersburg and 46.50 euro St. Petersburg > Helsinki. I also paid the 25 euro city tour fee, which is basically transportation to St. Isaac's Cathedral and counts as the "tour" you're required to take to qualify for the visa-free rule. In the ferry confirmation, I was told I would receive a separate confirmation for the hotel in 2 business days. I forgot about this for awhile and realized about 2 weeks later that I still hadn't received the hotel confirmation. I reached out to them and it took 2+ weeks, and several emails from me, before I finally heard from the hotel manager, who told me my hotel was booked and I would have to choose another, more expensive, hotel. I have no idea if it was actually booked when I made my reservation or if it was booked when the hotel manager finally responded to me almost a month after my reservation, but I was frustrated to say the least. It was at this point I asked if it was required that I book a hotel through Moby and was told that the "rules" had changed and I did indeed have to keep my reservation with them. It then took another 17 days, several emails, and a couple Facebook messages before I finally got a hotel confirmation at the new hotel I had selected. At this point, I was having serious reservations about the whole process, but I never considered cancelling as I had planned a lot of my 7 week trip around St. Petersburg and really wanted to see the city.
Fortunately, the hotel snafu was really the only issue I had with Moby/SPL; everything else was very smooth and every single employee I encountered was extremely helpful and spoke excellent English. I ended up leaving my phone at customs as I was entering the country and a very wonderful Moby Sales associate helped me extensively as I tried to find it, translating and just generally being comforting. Both of the check-in clerks were also very helpful and explained everything I needed to know and do and did a great deal to set my mind at ease about the whole process.
To be continued, running out of space....