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For a dry bag of some sort that you can put your main gear bag in for transport, the bags that are used for river trips can work well. I have experience with the roll up seal type bags, which work very well, and that is all that was used by the guides and by my self for my trip down the Colorado in the Grand Canyon. The link to Ortlieb bags that was shown above seems to feature the roll top type of bag, and that website is much closer to you than a US company would be. My bags came from a company called Cascade Outfitters, and here is a link to some dry bags: http://www.cascadeoutfitters.com/index.cfm?fuseaction=category.display&category_id=117&framein=
The Watershed bags listed in other links above use a little different closure, kind of like a Ziploc" bag, and those probably are fine, but I have no experience with them. The classic roll up seal has been in use by the military for many many years, and there is basically nothing that can go wrong with it. It just take a little more time to close, but not much more. But I do see that Cascade Outfitters also lists the "Ziploc" type bags now.
One issue is that many dry bags have a smaller opening at the end of a longer bag, and that small opening can make it very difficult to get a camera gear bag or small camera backpack in to the dry bag. If you want your whole camera gear bag to go in the dry bag, look for a bag that has a larger opening along the long dimension of the bag, rather than having a smaller opening at the end of the bag.
I set up my "day bag" with a shoulder bag for my gear, and then put it in to a dry bag so that I could access my gear easily. It became easy to un snap the end attachments and un roll the bag to get a camera out, and then stow it quickly before the next big rapids. I have not done an Antartic trip yet, but I am very sure you will want to take shots from the Zodiac part of the time. Of course when we got off the water and went on land hikes, I just removed my shoulder bag from the dry bag and used it normally.