Seasickness can happen to anyone-- even a Ship Captain.
I remember having a conversation with the Captain's wife, who said that this malady can occur unexpectedly; even to the most seasoned cruisers & even the Captain.
Fortunately, I have never experienced seasickness (even after 20 +cruises on Sea Goddess..now "Sea Dream." The roughest seas I have experienced was on the QE2 from the UK to NYC. This was a T.A. crossing in December. We loved it; sadly, many pax were quite sick on that North Atlantic crossing. OTOH, I have sailed from Spain/Portugal-U.S. and the ocean was like a lake.
The only time I felt queasy was when getting a massage in a small room. If considering a TA Cruise, I would strongly recommend a cruise which originates in Spain or Portugal and terminating in the Caribbean. Would also advise NOT using the gym (which may not have windows).
Fresh air is your friend. When all else fails, there is always Fernet Branca.