We went on a Baltic cruise in June 2004 and the North Sea crossing was perfectly calm but you can never guarantee the weather!! My wife is very prone to sea-sickness but was OK on this trip. However, when she does succumb, she generally has an injection which tends to make her very sleepy for 24 hours but she is right as rain thereafter.Despite being so prone to sea-sickness, my wife has never let it put her off cruising and we continue to be frequent cruisers.
I was on a cruise in the baltics last July and although the weather was not good (wet, cold and windy just like it was in the UK) the seas were not rough so seasickness was not a problem, However if it is there are various things you can use from tablets, to wrist bands to injections from the ship's medical centre if you get really bad. Cant say you wont get rough seas but dont let it put you off cruising. Even when it does get rough, not everyone gets ill and it certainly doesnt last the entire cruis.