A recent post on Duckworks Magazine’s mailing lists points to a new underlayment material named IntegraPlyâ„¢ (scroll down a ways). Exterior glue, hardwood, minimal voids, and the poster says that he saw it for around $15.00 being sold for underlayment. Might be just the thing for your small craft build.
Some of the other products on that page look interesting too… birch faced ply, etc. You might remember that I used MDO for my hull and cabin side build (and have so far been very pleased), so alternatives to traditional marine ply are something I keep my eye on.