David, assume a simple fiberfrax felt/stainless foil overlay on the engine side of a firewall.
First, the perimeter of the firewall gets a filet of 3M FireBarrier 2000 to close the small openings created by the fluting slots (gas seal...important!).
Fiberfrax felt and stainless foil are fastened with stainless pop rivets.
The entire perimeter and the openings around all subsequent fitting penetrations are sealed with FireBarrier.
So how does the water get in?
I'd argue it does not...and I've been flying such an installation about 400 hours now...in a climate not much different from Houston, or parked outside away from home, etc. It's been durable too.
Did your water test involve batting or felt? Was it notably hygroscopic (absorbs moisture directly from the air), or were you introducing liquid water?
That said, I have no particular objection to replacing the fiberfrax felt with cork sheet, assuming it performs as well when subjected to 25 sq inches at 2000F. Even if it does not perform as well from a thermal standpoint, a cork sandwich installation on the engine side is unlikely to cause harm.