Germany to Helsinki -advice needed

Hi all
As part of an RTW trip we need to get from Berlin to Helsinki for onward travel to St Petersburg.

Questions are
Has anyone done this, is Rostock Ferry to Helsinki the easiest route?
Any other route suggestions? - train to Stockholm then ferry?
Any recommendations for company's to book the ferry through?

Not looking to fly as we are over landing as much as possible.

Thanks
TQ

I took an overnight train from Stockholm north around the bay. Passenger service ends a couple hrs short of the border but the bus is included in the ticket price. You switch buses at the Finland border go to Kemi where you catch the train again the following morning for Helsinki. Nice trip if you have the time.