Hi Andrew,

I would have used a closed cell foam for the reason you mention, it does not absorb water.

Several sources, any good boat-builder should have sheets of it. A surfboard manufacturer/repairer or probably the easiest is to go to one of the composite material suppliers, FGI, Huntsman or Applied Composite Technologies and get good quality stuff, maybe like Durakore .... ask them for their recommendation.

Personally I would use the latter one, they have been in the business for long time and always seem happy to help.
There is my "two-bobs" worth.

The older of our boats has glass of about maybe 4oz on the inside and 2oz layer on the outside, as Philip said, puts inside into tension and the external layer goes into compression and really does nothing much more than stop the outer ply layer from splitting.

Enjoy winter ... David