Hmm, I woulden't consider plaster a good material for such large parts like a boat. Small parts it would be ideal. I did use plaster for a few molds years ago making cowls and wheel pants for my scratch planes. You start building large molds from plaster you would definately have to internally reinforce the structure with wire or other materials, similar to how concrete is strengthened. Lots of extra weight on top an already incredibly heavy mold.
Composite molds made from fiberglass mat, or layers of mat and balsa sheet with carbon fiber are ideal, however just fiberglass would work very well and would last a LONG time. Good luck not chipping a piece of mold out while pulling out a hull on a mold made from plaster. It could be done the way you are thinking, however in my experiences the negatives outweigh the positives. Perhaps Dana from DaVette Boats could chime in a little on this topic.
-Scott