IPMS gave up the majority plastic requirement for their competition several years ago. Your plastic kits may compete against plastic, wooden, paper, resin, or even metal models. They are all judged to the same standards.
This is the web page for the IPMS National Contest Committee. It contains all sorts of useful information regarding categories, rules, and the competition handbook. Pay special attention to the last, for it gives a lot of information on what the judges will look for. I believe that category 407 Sailing ships might be the destination for unpowered barges (regardless of their construction material). Category 418 Boats would be the home of ships' boats, again regardless of their construction. However their ultimate placement is the decision of the entry checker and ultimately the head judge.
http://ipmsusa.org/NCC/NCC.htm
This is the website for the 2010 convention. The rules & categories should map 1:1 to the national rules. You can also contact various people on the convention committee regarding special needs which you may have
http://www.ipmsusa2010.org/