My husband and I are wanting to take a ferry from Helsinki to Tallinn on Wednesday,19th of September 2018. Should we pre-book or buy on the day? I have read other posts on this subject but they are a few years old so I would really appreciate your feedback /advice for September 2018. Prior to catching the ferry we will be travelling by train from St Petersberg to Helsinki and we wanted to spend a few hours exploring Helsinki before catching the ferry across to Tallinn. We therefore don't really want to be locked into a time slot for a ferry departure unless necessary. Many thanks for your help.