We're taking the St Peter's line that departs from Helsinki port at 6pm. We'll be arriving from Stockholm on the same day by flight. Now the flight options are either arriving early in the morning (~9am) or at 3:20pm, unfortunately there seems to be nothing in between. Is the 3:20pm flight too tight to make the 6pm cruise (assuming they'll probably close check in around 5:30pm)?
I think I foresee a wonderful day of touring Helsinki in your future.
We'll spend a day in Helsinki after the cruise returns from St. Petersburg 3 days later, and we'd prefer to spend more time in Stockholm than Helsinki before the St. Peter's line cruise.
The 3.20pm flight is not too tight IF it arrives on time. It usually takes me 30 mins from touchdown to getting into the local bus and I always check a bag. I go through that airport about twice a month. You could then take a taxi from the airport to the cruise terminal. Or take the train from the airport to the main railway station "Rautantietori" and take a taxi or tram from there to the cruise terminal.