I searched their fares in September and again today - no changes for Gothenburg to Kiel. For those of you who have/will use their ferry, when should we book? Do their fares change like airlines, cabins become limited? Our pickup day is May 30, thanks!
Stena Line has multiple web sites - I booked one direction on the UK site and the return on the NL (Dutch) site.
It pays to check different (national) sites as well ... as Stena has discounts sometimes in certain markets. I got half off the base price about a month in advance of my trip with a special offer on the Dutch site.
You also may want to consider the different categories of booking. The lowest fare I believe allows no changes ... pay a bit more and you can make changes.
As Bob noted - may be best to book well in advance, especially if you're traveling during any busy periods. We took the trips at the start and end of Oct 2015 and it really wasn't all that busy - not sure what to expect at the end of May.
Both passes we reserved a room for 2 - and got a room for 4 (two beds either side and two more fold down bunks above that). We pre-purchased meals (both dinner and breakfast) on the Göteborg - Kiel run ... and on the return from Kiel to Göteborg we had some left over lunch that we just brought to the room and had that for dinner.
Going southbound the boat was about 90 minutes late getting into Kiel (may have been weather - it was a bit foggy but the seas were not rough) and going northbound it was on time.