OpenCPN
A friend suggested that I look at OpenCPN, an open source navigation and charting application from SourceForge. OpenCPN supports many chart formats, including ENC. NOAA provides free ENC charts for the US coastal areas and the Great Lakes.
This seems to be a winning combination: a free navigation and charting application, and free electronic chrts for most of the US waterways.
Can't wait to get out on the water to try this out!