I was just looking, Fry's has DesignCAD 3D Max for 59.95...

There well may be other CAD programs that are as cheap or better, but I know that this program does surfaces very well and that's what you need if you are doing a boat. I don't recall if it has assemblies and part files separately, but I think it does. You need that feature if you want to easily break the finished design down and make drawings of each part.