Just before the onset of the pandemic, I took the ferry from Stockholm to Helsinki. On the Silja Serenade. A very enjoyable voyage. The sailing through the Swedish archipelago was magnificent.
While I cannot recall what class of cabin I had, it was (1) an outside cabin with a window, and (2) on a higher deck. The window was moderately large and afforded good views. But most of the time you would want to be on the sun deck or the bow or aft decks for viewing. How much time will you be in the cabin other than for sleeping? I suspect, not much. Unless the weather is bad, you will want to be outside.
A suggestion for dining - skip the buffet. Yes, plenty of food and a good value, but also plenty of people! A better choice would be one of the restaurants. We dined in the Italian restaurant- very good food, service, and less chaos. Worth the few extra dollars/euros. Same advice for breakfast.