We cruised the Baltic at about the same time of year (Galaxy ex-Amsterdam) in 2001, and the weather was mostly excellent, temperatures in the 70s and 80s, but we had the odd cool day, and a little rain in Helsinki. Basically, this is normal Northern European stuff (we are British), so the advice to take layers is sound; I would also suggest packing a small folding umbrella - we take one most places either as a rain guard or a sun shade.
By the way, to see as much as possible is pretty hard work, but is well worth it.
We did the Baltic cruise on another line (Norwegian Dream) summer of 2000. It was hot everywhere we went on land-the evenings on the deck were cool and pretty windy. That was my favorite cruise out of the 9 I've been on-one that I would gladly repeat again.