I just got off a Cruise a few weeks ago( 7 Day Mexico on Princess), this was my first Cruise in over 25 years. My wife continued to Cruise during my 25 year hiatus and racked up those Sea Days. Anyways, I had a Great time...the ships have become much larger (25,000 Ton to 146,000 ton nowadays) almost too large really. Just the same there is so many things to do and see while on board. I wouldn't be too concerned about motion sickness...its hard to tell your out in the water.
OK..I won't be long winded here but GO...its Fun!!!
On Edit....if you can go at a Moments notice, check out "Vacations to Go" (vacationstogo.com)...great prices!