I also live in a wet area (Florida) and the down has worked great here and also when we travel up north as well. I use the watershed bag to keep it dry when traveling, camping, etc.. and it's overkill but if we ever had any floods or a Katrina type disaster all the gear will remain dry. I can even dive down to 300 feet in salt water with that bag and still have a nice warm bag. I see what you are saying about the syn vs down but I've been in the synthetic bags here as well wet and the bag was just as useless.

Here's a good comparison how down vs synthetic when it gets wet.
(The down bag was under water for 4 hours and was used)

http://www.trailcast.org/podcasts/trailcast-17-Jan-30-2006.mp3