Alternate hull skin: Painted On Fabric -- Most boat builders seem to use fiberglass and epoxy / or polyester resin to coat the underside of their hull. Some only use strips of fiberglass along the seams of the hull. If you were to build a really cheap boat such as a brick, and were considering not using epoxy or fiberglass at all, the next step up from no fiberglass would be to paint fabric onto the bottom of you hull. Best to put on a coat of paint, and then quickly apply fabric while still wet, and apply more paint to saturate the cloth. This technique is not as good as fiberglass, but better than bare wood. Also consider the technique for where you walk in your boat, makes a more durable surface.