I think one of the brand is call Trex. It is made from saw dust/wood fiber mix in with thermoplastic resin and press in high pressure to shape. It is much heavier than most wood due to the high content of plastic. It is made for deck. Trex is weaker than even redwood. Span for decking has to be reduced from 16 on center to 12 inches. And DO NOT believe that Trex does not rot. It is made from wood fiber, so it will rot whenever wood is exposed to water. I have seen mushroom growing out of it.
Whelter Trex is worthwhile for boat work, it think it is best left to your imagination.

: I was at a lumber yard yesterday and saw pvc "boards" made for
: trimming out houses. The dealer said it milled like wood (there were crown
: moldings made from the stuff- it did mill well. Has anyone ever used this
: stuff for boats? It came in 18' lengths, 3/4" and 1" thick, (and
: had no knots!) I don't know how tough it is, and it was not necessarily
: cheaper than wood, but I though it might be interesting. Any thoughts?
: Also, if you are in CT or RI, this place had great supplies of clear cedar
: and pine (some type of pine grown in New Zealand- farmed- clear long
: lengths. It is harder than eastern white pine, and I think a little
: stiffer.) The place is called Liberty Cedar, near the URI campus.
: Mark