Here's follow-up for anybody reading this thread: We bought and
installed the Garmin GPS 152. Once we got it going, we could download
and read navigation data to either our C-MAP program or to MaxSea,
using NMEA format. It turns out our C-MAP software doesn't support
uploading waypoints, but MaxSea does, and once we figured out how to
get the software to find the serial port, we were able to upload
waypoints fine, using the NMEA for that as well. This was just as
well, because MaxSea doesn't support Garmin interface formats (several
other manufacturers, yes; Garmin, no).

Thanks to everybody for your help!

Rob