We sailed up the UK coast/North sea in August and spent a night in a force 10 gale ... but returning a week later it was totally still - so who knows! Just take decent travel pills with you, and make sure you are sailing on a ship which handles rough weather well!
The North sea can be rough at any time of the year but you would have to be extremely unlucky to have gales in the summer months. Once you are sailing in the Baltic Sea you should expect no more than a gentle swell for the majority of your holiday although you my find that for couple of days a stronger wind may lead to some movement onboard ship. On a personal note I have always enjoyed any cruise I have been on to the Baltic including those to the Xmas Markets in December when the weather is decidely cold!
We cruised to the Baltics on 28th July (NCL Jewel) we too were worried about rough seas having never cruised before. However the first 5 days were so calm we could have been on a lake!! The next few a very slight rock then on our next to last day coming back across the North Sea a Force 9 gale! The boat did rock but nothing to worry about; we still managed our meals and enjoy our cruise. I took Stugeron which were very effective. A lovely cruise, itinary & highly recommendeded.
We have just cruised the Baltics in July and we had no rough seas at all. We sailed from Dover. I suppose its pot luck with the weather.
With the climate change, guess it's pot luck anywhere.
Web did the Baltic in Summer 2006. Baltic at St P was rough - yet coming back down the North Sea it was like a snooker table!
We sailed the Baltic a few years ago on the Royal Pricess as she was then, and loved it. We sailed in June and it really was a mill pond, and on reaching St Petersburg it was 93f. We had a balcony which we made full use of as the weather was so good, and the views were beautiful. Not sure about the rest of the year, but June for us was an excellent month to have sailed.