tallinnsilij ferry from Riga to Stockholm on a weak day in mid May

We are going to Stockholm in mid May. From Stockholm, we'll take any ferry to Tallinn,. Since both viking line and tallinnsilij ferry are going to Helsinki and there are plenty of connection from Hel to Tallinn, and we are traveling on weekdays, there is no need to book in advance, right? From Tallinn, we'll take the bus or train to Riga, since tallinnsilij ferry is the only ferry to Stockholm, I am wondering if we have to secure the ferry tix now to get a cabin and a better price? I have been getting different answers so it's confusing.