Agree that you contact Quark. On my trip, we arrived into Ushuaia early in the morning - we were already docked when we got up for breakfast. I had enough time to spend the morning wandering around Ushuaia for last minute souvenirs before heading for the airport. There's a place at the dock that held our luggage until we were ready to go.

I'd definitely leave some buffer time - you never know what's going to happen on the Drake passage. I had the Drake lake on the way over, but my trip back was rather "exciting".