: I think I read someplace in the distant past that interior plywoods were made
: with exterior glues.

John,
Most construction ply is made with the same types of glue, generally termed exterior. Waterproof glues have to pass tests involving repeated boiling and oven drying. Some exterior ply will pass, some will not. Some cabinet grade plys use glues that aren't as resistent to moisture. For whatever reason luan is generally made with superior exterior glues and will usually pass a waterproof test.

: The other thing I soaked under water for two weeks is blue building foam.

There are two general types of plastic foams: closed cell and porous. The expanded foams, like styrofoam beadboard, packing foam and spray urethane are porous and will act like a sponge. Extruded foams like styrofoam billets and sheet insulation are closed cell and will absorb a minimum amount of moisture. While I agree with Paul that moisture on the surface of foam is a concern, I think epoxy encapsulation should protect your wood from this moisture. If it still concerns you, it can be avoided by encapsulating your foam in glass and epoxy. It sounds like the foam that Byran is describing is extruded styrofoam billet often used for dock floatation.

Dow developed styrofoam for the production of naplam, the foam when dissolved in benzine and gasoline, gives naplam its viscosity and lovely adhesive properties. Styrene in the form of foam billets dissolves and mixes better and much more safely than the heated plastic. Given its origin, the cell structure of billet foam is more coarse than the insulation board. Thus the air to plastic ratio is higher, making it a better floatation. But it also means the open surface cells will hold more water. The coarse cell structure also causes the billet foam to suffer greater environmental degradation than the insulation, although both are effected by UV. Billet floation left in contact with water frequented by outboard motors will dissolved where the gas/oil mixture on the surface of the water meets them. In a few seasons this can result waterline erosion inches deep. If you can keep it out of sunlight and solvents it should perform very well as floatation.
Geo. C.