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.
Enjoy it,
Carol & Richard