Keep in mind that great stuff is polyurethane foam. The same stuff that they make life preservers with. It is extremely buoyant!! If it is going to be submerged, make sure it has a good bite on whatever it is using to hold it under water. I had to tear down my 125 paludarium because I forgot to plan ahead for that.

And standard definitions of water resistant and water proof don't really apply to the poly foam. I carved, it is waterproof. The outer skin provides a water proof barrier. But if it is carved, water can seep through the air pockets. It won't absorb water, but it won't hold it if carved.