Predicting weather and sea condition's is never going to be 100%. Sometimes a transoceanic crossing can be as smooth as a lake, other times really rough. Even a short crossing like the Irish Sea can be very rough. Most people that are prone to motion sickness can prevent it, by taking meclizine starting a day or two before embarking. It is not as strong as dramamine, but does not make you nearly as drowsy.Ever since the young Thunberg crossed the Atlantic on a ship (no, not a cruise ship, but whatever), I have thought either her way or a cruise ship might be fun. But I am prone to motion sickness on buses and as a passenger in cars, especially on curvey mountain roads and some of the very few times I was on a boat/ship on a large body of water. Would any cruises next to a shoreline be the same with regard to seasickness as crossing the ocean? How do they compare in that regard? I wouldn't want to try a cross-ocean ship trip without having a way to get back to land if any arising seasickness couldn't be dealt with properly.
If you still are concerned, you can always take a riverboat cruise on the inland waterways. They go thru nice areas, and not likely to cause seasickness. The only problem is they cost more than twice as much per day, as a typical ocean cruise...