Only two weeks back from a cruise that went up the North Sea from Southampton and we actually experienced pretty rough conditions going out during the sea day (yes, I did witness people randomly being sick). Sadly it was on the gala night and it kind of undermined the sense of occasion. Even the crew told is was the roughest it had been in a while. And worse still we couldn't actually make into our first port and just had to sail on.
The good news was that on the return via the same route it was like a mill pond and totally calm with lovely weather so as suggested, I think being ready for anything is not a bad idea.