December may be too late for Peninsula Valdez. I went mid-Nov and most whales were already gone. Elephant seals were in mid-molt. Penguin chicks had hatched. While the penguins will still be there, the others will be reduced populations. The weather will be very nice. Ushuaia will be warming with the pending start of summer.

I can provide additional info on my experience if desired, but not while on my iPhone - as I am now.