I agree wholeheartedly with both of the other two posters, but then again, I'm a bit prejudice on the matter.
A good cruise specialist can be worth their weight in gold as they can help answer all your questions and make your life alot easier with less hassle. Of course, it's always good to find one who's 'been there done that' so they can also help with questions about the ports of call.
And travel insurance, especially when traveling overseas, is a must and we would never think of going over there without it.
As for the Baltic cruise, my sentiment is longer is better! We did a 12-nighter on the Holland America Eurodam and it was awesome!! Then again, we also did a 2-1/2 week tour around Europe to go along with it, so we were gone for 32 days. But our cruise started and ended in Dover, so we were able to take the train to London and visit that wonderful city for 4 days.
We've been to all the ports you'll be visiting and I can say that Tallinn, Estonia was a very nice surprise. Great place to see.
Also, when visiting St. Petersburg, I highly suggest doing an independent tour with a good reputable company - there are several to choose from, but we used SPB Tours and they were wonderful. We paid roughly the same as what the cruise line offered, but we only had 10 people on a brand new Mercedes Benz van whereas the cruise line tours had 45 people on a big bus. Gee, guess who had more fun with that one! The smaller group is definitely alot more desirable, especially when visiting the museums. But we absolutely loved St. Petersburg - very steep in history and amazing places to visit. This is one place you do not want to scrimp on your shore excursion as you'll want to see it all.