Today's modern ships are equipped with stabilizers that significantly reduce the motion felt on board.  So if you get sea-sick on small watercraft, it doesn't mean you'll get seasick on a cruise ship.  Chances are you won't!

However, there's always the possibility of encountering rougher seas where motion will be felt, and there are some people who are extremely sensitive to any motion.

If you're concerned about sea-sickness, you can get medication from your physician that will relieve any symptoms you might develop.  Medication is also available onboard the ship.