Weather stripping will block heavier water, but nothing short of sealed plastic will stop moisture in new England. Air is too wet, dew points etc. Even dry bags will get damp over time.
Ziplock makes oversized bags:
http://www.ziploc.com/Products/Pages...NonFoodStorage
I'm a fan.
At the very least I carry a roll of contractor grade garbage bags so people with loose clothing, jackets, bags, etc can keep them dry in hatches.