I took the Tallink ferry from Tallinn to Helsinki a few years ago. It's a large ferry, and I booked it near the last minute. This company uses the west terminal in Helsinki vs. the main one by the main harbor near the center of town. Other ferry companies may use the main terminal which could be easier for you to get to. I was immediately taking another ferry out of west terminal anyway so this worked out well for me, but you can take a tram (I did this a few days later when I returned to west terminal) between the town center and west terminal, very easy but just one more extra thing and little expense.
I spend most of the time on my ferry trip on the top deck enjoying the scenery. I got the cheapest ticket available. It was an OK ride but not much to see between cities. I think the scenery by Helsinki might be better if you depart/arrive using main terminal.