phil, good points, but you forgot medex. I have used this in the housing trade for exterior treatments around doors when people want fluted columns and the such. expensive for a sheet, but works just like mdf, however has some water RESISTANT properties. notice not waterproof.. still need to have some treatment, but will outlast mdf 10 to 1.
it can be used, but the precautions I would take are to treat the entire box with some water repellent and ensure it's in a location that won't see direct contact and raised off the carpet..

btw, sweet boat