Our first cruise was a transatlantic in Nov 2003, on the Splendour, Lisbon to Galviston. The weather wasn't the greatest in the world but it was on the whole good. I'd say take some warmer clothes for walking on deck at night.
One thing I would say is be perpared for a change in the itinerary. Ours changed at the very last second due to rough seas, but you just go with the flow, no pun intended ;-)
Have a great cruise