We did a transatlantic trip on QM2 (Southampton to New York) June 2-8 this year. We had all weathers from gales, fog, rain, sun to a record (& unseasonal) heatwave of 102deg in New York! We didn't sit out on the open deck apart from after we docked in NY waiting for disembarkation. We used the outdoor pool though, which was pleasantly heated.
It always seemed breezy on deck, even in the fog. Was it due to the ship's speed? I don't know. I noticed that a few people sat out for a short while, often with a knee rug. It was fine for walking the decks - take a fleece or something suitable. The ship's Officers close the decks to passengers if it gets too windy/wet.
We had a sheltered balcony cabin (no 6208) & occasionally sat out there for short spells. It warranted a cardigan/jacket though.
The sea conditions varied from dead flat (in the fog) to rough (in the gale). But we didn't feel any movement. It's a fantastic ship & she just cuts through the waves without any fuss. We had a wonderful time & I can't wait to do it all again!