score:3
Walk-up fares will generally be expensive (and almost never cheaper) than booking online, so as long as your schedule is fixed, I would recommend booking ahead.
Also check out day cruise fares: odd as it may seem, it can be much cheaper to book a return fare, which are often steeply discounted, than a one-way fare, which usually are not. For example, at time of writing Tallink is offering cruises for €12, whereas same-day walk-up fares are €31-41. And yes, it's fine to throw away the return.