As a frequent traveller with Tallink Silja it's very easy for me to answer this question.
You must get a physical paper key with a magnetic stripe for your cabin and to be able to pass the automatic gates at the terminal. You can get the key(s) either at the manned kiosks or using self-service machines.
There's no need to "check-in" before you get ...
Yes, it is legal to feed pigeons in Sweden.
However, it is not legal to throw things other than chewing gum, cigarettes and snus on the ground. So you can't just empty a 100 liter bag of bread on the street.
Also, some places like cafés and restaurants ask the guests not to feed the birds. One should follow these instructions as some of these signs ...