Is this a one-way trip or do you intend to return to Helsinki?
Which ship are you thinking about, the Silja or the Viking?

You're from the North, but even you might be surprised at the length of the twilight and dawning at this latitude in the summer. At the end of August you'll still be able to see the islands on the night crossing.

There is a train from Helsinki to Turku harbour timed for the morning sailings.

cheers,

Henry