The Straits are inland, so less rough than the open ocean as far as swells, but are notorious for very high winds, so there is a lot of chop. Not so noticable on the ship, but certainly is on the tenders at Puenta Arenas! The Horn is variable...this is why it is so famous. It can be windy and very rough one day, and cold and like a lake the next. We were lucky, and although it rained all day it was clear enough to see the Cape well, and we had some good swells but nothing unusual. The Falklands are the same. Sometimes it is so rough they don't even let anyone off on the tenders, and other times it is like a lake. I lucked out on a "lake day" in late November (Thanksgiving Day as a matter of fact) and it was even warm (unusual here) enough to wear just a t-shirt. They are also notorious for high winds and lots of rain here.
We had our roughest seas between Valparisio and Puerto Montt. I don't get sea sick usually, but I did on this trip in that area.
It is best to be prepared for rough seas and cold weather in the further south areas of this trip. This means extra packing, but that is better than being cold. Once you head north again you can pack all your cold weather clothing and be ahead of the game for disembarkation.