I don't often come on the "cruises" board (and obviously not many others do, considering this is one of the most recent threads!) but I had to comment.
I've done 4 cruises out of Southampton now. 2 Mediterranean, 1 Norwegian and 1 Canaries. 2 in May/June time and 2 in September.
Obviously the Med and Canaries cruises involved the Bay of Biscay. Bearing in mind I've crossed the Bay 6 times now, I've never experienced it rough!
Worst seas have been in the Med near Barcelona and in the English Channel on the way home on one cruise. That was the only one that's ever made me feel queasy.
The North Sea was very, very windy but the ship movement wasn't too bad.
Top tip though which certainly is true for Royal Caribbean cruises (not sure about the others), but they give out free sea sickness medication if you need it.
Do you get sea sick on other boats? I don't generally but have been known to feel sick on smaller boats that bob up and down. On our Med cruise that was really rough loads of people were sick but I felt absolutely fine. If you're OK on, say, a ferry, then I think you'd be fine on a cruise ship.
Cruise ships have stabilisers to stop the worst of the pitching from side to side, but they don't work on the front to back waves so it's luck of the draw really where the seas are coming from.