Be aware that Antarctica is NOT cold in the summer, which is when you'll be visiting – at least not on the peninsula where the tourist ships go. The coldest that you're likely to encounter is -5C, which is warmer than New York on a typical winter's day.
Just keep your gear under wraps when moving between ship and shore by zodiac. A dry bag is OK, but likely overkill, and can make getting into ones gear slow and awkward, especially when actually shooting from the zodiac. A backpack and a plastic lawn trash bag to protect against spray is likely all you need.
You might want to consider a Kata rain cover for your camera. This will protect against blowing snow, and rain, which is just as likely in some locations.
Michael