Yep, I purchased Canvas for professional use (~$300), but Canvas also happens to be extremely well suited for doing deckplans for many reasons including being able to draw in scale and the display of lengths and areas in scale units.
That said, I've seen some outstanding deckplans using whatever the person happened to have handy (or came with their computer) including AppleWorks and MS Paint.
As mentioned, CC-Pro and Cosmographer Pro aren't all that expensive for commercial drawing packages. I've seen good plans done with Fractal Mapper, which is cheaper.
If you want free, you might try OpenOffice.org. You won't get the scaled drawing capability of the commercial packages, but it is free.
Ron