We did the Baltic last year in early June and the weather was very good once we got out of the North Sea and into the Baltic, same coming home arrived in all the rain we had that so called summer. Our last trip was to Ypres and the weather was excellent, in St Petes is was on average around 18degress most of the tri.
It can be pretty variable. I was in the Baltics in july and it was wet windy and barely got above 15C. However this was jut a reflection of the awful summer we and North Europe are having this year. I have been in Scandinavia at other times and it has been bright and sunny and warm. The weather can be pretty similar to the UK. My advice would be go prpepared for anything! I usually go for the layer approach - rather than take really warm clothes, i take lighter weight clothes and then wraps and cardigans that i can add if it is not as warm as I hope for. Also take a rain coat and umbrella. BBC teletext or various websites give 5 day forecast for most European major cities, so you could also check this out the day before you go when you are doing your packingHope this helps and that you are more fortunate than I was with the weather.
They're quite right take something of everything particularly as you has no flight luggage weight to worry about. It is one of the benefits of sailing out of and into the U.K. Hope you enjoy it, Neil Dow.