I am wondering if the water runs thru the aluminum housings or if it is going through some lining? It would seem that the salt water would eat these housings up.

Yes it eats them plenty fast,
we take ours off and run a coating in them to slow it down.

The Eickert bell housings from Hardin Marine are hard coat anodized to help with the corrosion.
Eddie

So are the cores Stainless steel? If so would there be a market for a cast stainless housing? I have cast SS parts made all the time.

Cast ss that would be cool cost factor ?