In 2016 it was fairly rocky because the polar vortex in the US was wreaking havoc on the currents. We barely made it into Castaway (and had we not had Commodore Tom I have my doubts as to if we would).
Bottom line is, you are on the ocean which is always moving, and at any point it could get rough. Summer 2016, I was on
RCCL on a 12-night Med cruise. The final night we had to cut directly across the Med to get to Barcelona. The prediction was smooth sailing. Well, at around 2am we ran smack into a random pop-up storm that was in Tropical Storm territory on the Beaufort Scale. I had a bow facing cabin and could hear waves crashing over the bow. And we were rocking. On the usually reported as smooth as glass Med (I later learned it it notorious for these pop up storms). I loved it - the rockier the better so I know I am on the water (I would not cruise if I didn’t) - but a lot of people didn’t.