Mariann Hi tachyon, I just joined today and saw your post. I did Antarctica this past February. Saw you plan to stay in Ushuaia for a bit. I stayed there for 5 days which turned out to be 3 days too many. Also, I'm sure you know to layer, we had days were we walked around the deck in slacks and short sleeve tops and an hour later it was snowing.
Crossisng the Drake Passage is very very very rough so if you start to feel sea sick take something immediately. You could also bring some ginger tea with you and make that your beverage of choice. I never get sea sick and I got a little upset on the crossing.
As for diving there. We saw a lot of tiger seals which are vicious and have grabbed and drowned diver before including a research scientist just last year.
Having said all that, it was a trip of a lifetime and I loved every minute of it. HAVE A BALL!!!!
I'm now planning a trip for next year on the Trans-Siberian Railroad starting in Vladivostok, with a stop in Mongolia and ending in St Petersburg (not all of this is on the rail route but I'll make some side trips) and, of course, several stops enroute.