I recommend buying CD's and running your charting off
a laptop linked to a GPS.

There was a thread recently about scanning your own charts
if you want to do it cheaply.

http://www.oziexplorer.com/

wrote
What if any chart software can be obtained at no or lower cost than
the Garmin proprietary "Blue Chart" CD that can be downloaded to the
Garmin 78CS GPS?

We thought we remembered someone once saying that there is such, but
aren't sure and so would like at least to know whether we should stop
trying to look and will be stuck with buying the Garmin software.