The inside passage parts are smooth as glass. The other parts are not... The Alaskan was our first cruise ever, based on everyone's description of 'smooth' sailing we just had seabands for the kids and Dramamine Naturals ginger pills and some ginger candy. I was queasy the first night before dinner--getting my sea legs as it were, and then I felt fine the rest of the trip--I could definitely feel movement when we had it. DD12 and DH had no issues. DD16 felt queasy anytime there was a lot of movement--1st day, 2nd day, early on the 3rd day, and the last day at sea. (That last night in bed the ship was really rocking as well.) Basically anytime the ship was outside of the inside passages. That last day at sea DH and I did the Art of the Theme Show tour and our tour guide was holding onto the railing as she noticed the ships' movement as well--and when the CM's feel it, it's really moving. For DD16, on day 2, I did go down to the health center on deck 1 to get free samples of seasickness medicine. They have packets of meclizine in a bin, no need to see anyone for it. She ended up taking 1/day on the 'sea days' and felt better. If you bring Bonine (meclizine) and take 1 a day, you'll likely be fine.
I don't necessarily think it is 'really rough', but I do want to emphasize that it is not 'like glass' the whole time.