I know there are a lot of questions on this site regarding the St Peters Line boat cruise from Helsinki to St Petersburg (I, too, was looking for information when planning our trip), and since I just got back from a visit (using this line), I thought I'd write a review to help out others who may use this mode of transportation in the future.
First, we are Americans (living in Helsinki) and we had no problems as it related to the visa, so I assume, if you're European or any other nationality, it's just as easy.
First, we reserved our on-boat room, online and it was quite easy (http://stpeterline.com/en/OnBoard/News.aspx). If you plan on staying longer than the 10 hour one-day trip in St Petersburg (i.e. dock in the morning and get back on the boat that evening), you'll need to choose "return" trip, rather than round trip.
If you want the visa-free stay in St Petersburg, you need to stay less than 72 hours in St Petersburgs (from the time you arrive, not the time you get on the ship). We left on a Thursday evening from Helsinki (arrive in St Petersburg Fridaymorning) and got back on the ship Sunday evening...so about 56 hours total in St Petersburg.
The ship itself is simple. There are two restaurants, two bars, a cafe, a duty free shop, a casino, a pool and rooms. There is also a kids play area but it's only open limited hours. Pretty basic, but you're only on there to travel, so it is what it is.
The room itself is a cruise room. Basic. They do have baby cots on board. Our room, the Commodore, had a window and its own bathroom. Other than sleeping, you don't want to spend much time in there.
When we left Helsinki and arrived in St Petersburg, you have to go thru passport control in both (which takes the most time...lots of line and waiting in lines). so just be prepared.
Your boat boarding pass serves as your visa during your stay in St Petersburg. You will NOT get a visa in your passport, you simply carry around the boarding pass with you at all times, along with your passport, in the case you get stopped and asked for your papers. This is what you show the police, and YES it makes you legitimate (the boarding pass in your visa), so long as you stay less than 72 hours.
We had absolutely no problems during our stay nor during boarding procedures at either location. It was super easy and a great way to travel without having to go thru the visa process.
We did stay at a hotel two nights in St Petersburg (Comfort Hotel...which was wonderful, great location and highly recommended), and they could have provided us with Visa paperwork, but again, we did not need it b/c the boarding pass from the boat was all we needed.
I hope this helps. I would definitely recommend the boat. It was hassle free, easy, etc. If you have any questions, please don't hesitate to ask.