I'm with rappstar and Mike--I would go with down, especially for your situation. I have a zero degree Marmot and its just fantastic.
But, regarding synthetics, the new Climashield insulation is supposed to the latest and greatest. TNF uses it in their Snowshoe and Cat's Meow, 0 and 20 degree bags, and I've recently seen the Cat's Meow on sale at REI for $99. I've owned one and known others who've owned them and have been happy with the fit and feel.
If money wasn't an issue and I had to have synthetic I would buy a Kifaru bag.
I have, and known others who've had Wiggy's bags, and they are also tried and proven.
If I knew I was going to flip my raft or spend a week in the rain in Alaska I would absolutely go with the synthetics. Otherwise I would go down and take the xtra precautions to keep it dry.
If you are in so deep that you cannot escape alive because your down sleeping bag got wet, then you just might want to work on your survival skills and not take gear that makes up for that lack.