Helsinki to Tallinn ferry

Did you check ferry websites that sell tickets? I would think that with the way things have been, and will be, everything needs a reservation, especially ferries. That is a popular route so I would look into it. I checked directferries.com and they have 3 different companies servicing that route. You can read their cancellation policy. Or, you could research then go to the ferry’s direct website to see if that would be better way to book.

Yes, you should buy the ferry tickets in advance.

They are big ferries on this route, no need to book ahead
A warning about the small fast boats. I got a ticket for one of these before but was cancelled on the day due to high winds so had to get to the main ferry terminal which is some distance away

I bought our tickets online in advance for the Eckero Line, M/s Finlandia for our August 2022 trip. I don't like to be surprised if many people decide to travel on the same day. You just never know.

Probably no reason to book ahead though it might be slightly cheaper if you are able to commit to a specific time/day. There are several companies running ferries as noted.

FYI, the different companies use different landings in Helsinki - some use the one closer to the center of town, the others use the west terminal (as I recall, that's the one I landed at when taking the ferry in from Tallinn). It's just a short tram ride between the center of Helsinki and the west terminal...but make sure you take that into account.