We went in October, and it wasn't too bad. Plus we were on the back of the boat, and up on a higher deck, so we would have felt it the worst of just about any room on the boat. My wife and I had both gotten prescriptions for the transderm scop patch before we went, and we started off wearing them. After a couple days, I found that it was really causing problems with my throat being dry, and stopped using it. My wife used it all week, and neither of us had any trouble (she's much more prone to motion sickness than I am). We had a night or two where it was a little rough, but we're not talking 6 foot seas or anything.
My suggestion is if you think you might have a problem, ask your doctor for a prescription and start off with them. It's better to be overly prepared than to actually get sick and have a lousy time. The crew will have dramamine (complimentary) otherwise.
We are in the process of deciding to go on that cruise for our honeymoon and I am trying to get some reviews of the ship and service. I did hear from a travel agent that the seas in the area can be a bit rough that time of year so lets keep our fingers crossed!!