I'm considering to travel from Helsinki (Finland) to Stockholm (Sweden) on the direct overnight ferry on a Friday night on the Silja Symphony, but I'm worried about the reputation of those ferries as party boats. How loud does it get? Can I hope to escape the drunken, rowdy, howling masses if I book a B-class cabin on the ship? I can bring ear plugs, but if it's anything like, say, the night train from Munich during Oktoberfest, then those won't be enough to get any rest. What is it really like?
I took the Viking Line Gabriella ferry the other way from Stockholm to Helsinki last year. I went on a Tuesday night. Yes, people are having fun, but it didn't disturb my sleep at all and my ordinary cabin was far away from the music.
The entertainment is all organised rather than people chaoticly drinking and shouting in the accomodation area. Most people are fairly old as well, probably majority 60+. The vibe is nothing like Oktoberfest (I went there last year as well) and there were no rowdy masses.
Here's a picture of the bar on the Gabriella. Plenty of fun to be had if you like dancing to Abba covers, but the rest of the boat including the dining areas were quite chilled out.
There can be some noise and activity in the public areas of the ferry, like in the bar and restaurant, the sleeping areas are generally quiet and chilled out.
Of course, there may be occasional disruptions, such as passengers returning to their cabins late at night hammered, but these are typically minor and not disruptive much.
The short answer is not very rowdy, but as a rule of thumb, the more you pay for your cabin, the higher your floor, the older the crowd and the quieter your cabin. At the extreme ends of the spectrum, on Silja the cheapest windowless four-bunk cabins below the car decks are where the students party, while the top of the line Commodore Suites have their own dedicated section at the front of the ship where you can't even get into the corridor without a special key. But security keeps a pretty close eye even on the lower decks and anybody who's really out of control will be hauled off to the drunk tank pretty fast.
Back in the day Viking Line used to allow younger cruise passengers than competitors Silja Line and was thus known as the party boat, but these days I gather the roles have reversed and the age limits are largely gone.
The day of the week has a big impact on the number of passengers. In the weekend there might be more party people cruising. You can ask the ship's staff about the amount of passengers on board. Cruise and one way passengers might be separated into different cabin sections, even if the cabin class is the same. In other words, passengers in the cabin next to yours will be also traveling to Stockholm instead of just cruising to Stockholm and back to Helsinki.
For your own peace of mind, once you are in your cabin, you can find out how to contact the ship's information desk, which is the same thing as the reception in hotels. There may be a telephone in your cabin or you can contact the information desk using your own phone. If necessary, the ship's security guards will come to interrupt the loud cabin parties. They may also confiscate a bluetooth speaker etc. if necessary.