As a person with a history of motion sickness, I always hated flying, but could also get nauseated in cars, buses, trains, etc. Sea sickness was the worst, from riverboats, lakes, glass bottom boats, whale watch tours, sailing, up to the largest cruise ships. Caribbean cruises I was anywhere from queasy to miserable. Wrist bands and ear patches did not work. Alaska inside passage with balcony cabin was the best, very smooth. Last cruise I took I ended up with MdDS; 8 weeks of severe vertigo after getting off a 5 day out of Tampa. No more boating for me.
Mal de debarquement - Wikipedia
If you still want to try it, go for the following:
Less stormy time of year on the open seas.
Larger ships more stable.
Mid ship cabin, lower levels. Don't look at the horizon.
Upon arrival, see the maitre d' to request the most forward table possible. Dining rooms are usually located at the tail end where there is the most motion and you could be too nauseous to eat.
Bring dramamine/meclizine.
Try a 3 day for your first time. It's one thing to be trapped on a flight for 3 - 6 hours. 5- 7 days on a moving ship can be agony.
Some people love cruising, but it's just not for everyone. Not fun and a waste of good money if all you can do is sit quietly or lie down and not walk around coz you feel sick. If you get queasy trying to read or look at a map in a moving vehicle, that might help you make your decision. Good luck!