I've done a transatlantic on Celebrity a couple of years ago.
The seas can and sometimes do get rough.
That being said we didn't hit any rough seas on my cruise.
The transatlantic cruises that carnival offers are re-positioning cruises and happen at the beginning and end of seasons. Typically April and October. These months typiclly have more volatile weather, so the chance of having rougher seas is better.
As far as all the time at sea, if you like sea days on other cruises than you would like the time at sea on a transatlantic. I personally love sea days and I really loved the transatlantic I did.
I'm currently booked on a 16 day transatlantic on the Carnival Splendor leaving Rome on October 29.
I found that the crossing days (sea days) were a great time to relax and explore the ship.
Hope this helps you.
Brad in Ma.