Generally the larger the ship, the less the motion. We cruised on the Majesty of the Seas (since retired) and you could feel the motion continually: I didn't mind it at all (actually like it) but on one moderately rough day there were barf bags hung at the bottom of each stairwell. Conversely, on Harmony and Oasis, there was very little sensation of motion even when in rough(er) seas: Sitting in "Central Park" there is nothing really to even remind you that you are on a ship: you can't see the water from there and no perceptible sensation of motion of any kind.

Also, stabilizer technology improves over time: a newer ship will have better stabilization technology.

So--biggest and newest.

Ships movement
 

Correction to movement on ship.
The higher up you go especially towards the front the more movement that will be felt. Low and center is best for stability.

First, have you looked at the age distribution of any given cruise line. Holland America average age is older so has very few kids. Viking also does not cater to kids. Carnival is the opposite with many other cruise lines falling in between.

Second, the center of the ship lower floors moves the least. Take a rectangle, rock it back and forth. You will see that the ends move the most and the mid top will move more than the mid center. I agree with the previous posts on taking medicine. You can pick up Bonine at Walmart which works.

Finally, if you decide to purchase travel insurance, seriously consider a larger 3rd party company such as Allianz and purchase the premium policy. Last year, Vantage travel went bankrupt and owed a hundred million to customers. While another company bought the ships and offered passengers to use the credits towards one of their cruises, you could only use the credits for up to 50% of the cruise cost.

I’ve been on over 30 cruises, and have cruised most major lines. I’ve settled in with Holland America the last ten or so years, but still cruise occasionally on Princess and Celebrity. It really comes down to what is most important to you on a cruise as to what line you would most be happy with. For me, food and service and a relaxing environment trumps a loud “party” atmosphere. I don’t need carnival-type rides, or the big mega-ships that to me feel like a floating shopping mall. I like the mid-sized ships, where you actually “feel” more like you’re at sea. If you don’t need water slides, rock-climbing walls and glitzy, non-stop “Vegas” type atmosphere , then you’ll appreciate Princess.

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I literally get seasick on an float in a pool. I got sick on a fishing trip and did not go on any type of boat for 25 years so your wife's concern is real! The doctor can give her a patch and it works great! You shouldn't drink alcohol on it, which I don't . It does make you thirsty. The lower the deck, the less the sway plus being in the middle of the boat as much as possible is best. But I have gone on upper decks and had no issue, I live in the Boston area and people take cruises out of there but I would never cruise the Atlantic. You can take 3 day cruises out of Ft. Lauderdale or Miami to try it out.

My wife and I have sailed 2x in the last 5 months on the Enchanted Princess (eastern Caribbean and Western Caribbean; both from Fort Lauderdale) - we have only positive comments to make about this ship and Princess Cruises. Ship is relatively new and immaculately maintained. Food is fine throughout the ship; entertainment was superb, all brought to life by an extraordinary Cruise Director. We have been to most ports in the Western Caribbean, so we decided to stay on board in some of them since we consider the ship the primary destination. The "Medallion" system on the Princess ship worked best of all the Princess ships we have sailed.

I’ve cruised on Princess 19 times. They are my favorite cruise line. The Enchanted Princess is spectacular and you will love it.

I’ve never cruised Princess and don’t know what your favorite line is. I sail mostly Celebrity and Holland America. Enchanted Princess is one of the “favorite” ships and I’ve never met anyone who’s had negative things to say about Princess. I’ve met quite a few people who often sail Princess along with the other 2 I mentioned. If it’s an itinerary you want and the price is right, you should do it.

I get motion sickness in cars and rides etc but never on cruise ships. Get a cabin on a lower deck in the middle of the ship. I’ve never really encountered anyone who’s been seasick on a cruise ( except for rough weather on crossings etc). Those patches and wrist bands seem to work for the people who wear them. I’d tell her not to worry and try a 4-7 day cruise to start with.

You can get a patch to wear behind your ear from the doctor for sea sickness.

I've sailed with Princess 7 times. I've also sailed with Royal Caribbean, Carnival and Margaritaville at Sea. We loved Princess in the beginning but the last two times we left disappointed. The ships are generally beautiful but the entertainment - especially since covid - is greatly lacking. Now, keep in mind that this is subjective and everyone has their own opinion. If you like a quieter atmosphere then Princess and certainly Holland America would be a good choice for you. We have another Margaritaville at Sea cruise coming up in October and we will be on Oasis of the Seas in January - can't wait!

I cruised on Ruby Princess in January. The food was about the same quality as the FMK-run country-club restaurants we have here in TV. Their specialty restaurants were not us good as the higher-end restaurants we have in TV. I was very disappointed.

The entertainment was very weak compared to other ships I’ve been on. There were very few options.

There were very few children on the late-January cruise (I saw maybe 10) and they were very young. I guess that’s because schools were in session.

I liked that I could use their app to order food/drink and the medallions we wore would lead them right to me—even at the pool. Very convenient.

I used their WiFi, which used SpaceX’s StarLink and, although very slow compared to home internet, was good enough to stream video. But, I waited days for a 1 GB file to download and finally cancelled the download. Best ship WiFi I’ve ever had.

I have no desire to cruise on Princess again. My favorites are Celebrity and Holland America. I’ve also cruised on Royal Caribbean, Viking, Disney (a nightmare!), and Hurtigruten.

I'm currently on a 27 night European cruise in the Caribbean Princess. It's my 8th cruise on this ship and my 16th cruise on Princess.
As others have said, it is very subjective when it comes to rating a cruise. We enjoy Princess because it's not a party ship or children's playground. The food in the dinning room is good, not great. After the 3rd week it seems to be repeating the menu. Princess still services the staterooms twice a day, while several of the other cruise lines have gone to only once a day. They don't charge if you want an extra meal at dinner, something some other cruise lines are now doing. The cruising now is different than it was before COVID. The Cruise lines lost billions of dollars and their stock still hasn't recovered. I guess it's understandable that they have to recoup their losses some way.
If food is important than Celebrity is a good choice. Their Edge class ships are amazing. If your a true foody, Oceania is a great choice.
Virgin is unique in that everything except booze is included in the price. That includes gratitites, internet, specialty dining and some physical fitness classes. They are adult only. Don't let the hype fool you. The average age of people on my cruise were more over 50 than under. They don't have formal dining rooms. All their restaurants are specially restaurants. Everything from Italian, Mexican, Seafood, Steak, even Vegetarian/Vegan. We found every meal to be outstanding.
As you can see there are many things that makeup a cruise. I've only touched on a few of them.

A “Princess” cruise was our first cruise, and still our favorite. Albeit that was 18 years ago and a dozen cruises since. Things change. We prefer smaller ships. They used to have old re-runs of “The Love Boat” playing on one of the TV channels which we found fun (filmed on Princess).

We cruise NCL & Princess (15 cruises); We prefer Princess for ambiance & staff but Princess for cabins and amenities (on ship laundry facilities especially on a recent 34 day cruise). I prefer NCL entertainment and food.

Some ships are better than others.

Motion sickness tablets (Bonine comes to mind) are OTC. Scopalimine patches are prescription only. Ginger (at the bars or sushi places) or even ginger hard candies are helpful.

Bon voyage!