Wow... how exciting... CONGRATS...
I'm all in favor of taking kids on vacation.  It is what we have been doing for years.  We've never left our children at home and probably never would.  We don't have grandparents to help out so, we can getaway...  I think, cruises are great for babies.  They have lots of fun.  But personally, I wouldn't take a baby on a cruise to Europe.  It just wouldn't be fun.  Many ports in the Mediterranean were not stroller friendly and I'd imagine the same would be in the Northern Baltics.  I remember this one family on our Rome excursion.  They had a baby in a stroller and the wife got separated from us a lot.  Because she had to take a different entrance.  We would go up stairs and she would have to go around the back.  Anyways... that day the dh and dw argued a lot.  It was quite embarrassing...  I'd start off with something smaller like a 4 day cruise.  Or do something like the transaltantic cruise where you'll be on the ship the majority of the time.  If I remember correctly, many of the excursions did not even allow infants.  So, you really have to think about how much fun you want to have and how much money you want to spend to not have as much fun as everyone else.  Hope you don't take anything I said with any offense but I just wanted to give you a different prospective.  And, trust me my kids have been everywhere with us.  We've taken them, on 6 cruises, we've been to Hawaii, Mexico, Florida, Williamsburg, the Oregon Coast, Newport Beach, San Diego, Las Vegas, etc...  In 2007, we did the Mediterranean cruise and they were 6 years old and 9 years old.  The 9 year old was at a perfect age.  The 6 year old was iffy.  We we do the Baltics they will be 12 years old and 9 years old.  I can't wait!