Thanks for that answer Meindert. It is as I suspected, and I am
somewhat annoyed that the RayMarine gadgetry doesn't automatically talk

NMEA. to other components. I thought the entire reason for NMEA
standard is to have different brand components communicating.
Raymarine has ignored that.

To say it that way indicates a level of ignorance about the issues involved.
Raymarine certainly didn't "ignore" anything. For what they wanted their
devices to do they chose to use something better suited than just plain
NMEA.

If you bought stuff that doesn't do what you want, then who's to blame here?

My Garmin has wiring that says "DSC". Can I assume that it will work
just by wiring it into my ICom VHF?

Start by asking ICOM. They'd certainly be a position to know.

This stuff is fascinating, but it doesn't seem like there's much
information available about it. Do you know of any books on this
subject?

The market for nav equipment is considerably smaller than other technology
products. As such there's not much written up about it.