The problem is across the entire marine spectrum, not just pleasure craft.
There are thousands of commercial vessels that not only bus the nav gear
through NMEA and other IMO approved interfaces,but now also host Ethernet
networks as well.

Thousands does not equal economies of scale typical for computer electronics
markets.

That and, iirc, ethernet has no standardized connectors for watertight
fittings. Then there's the hassle of all the wiring having to be home-run
back to a switch. There's no way to daisy-chain the instruments along a
single backbone. So it's more wire to break, more connectors to leak. No
thanks.

The IMO is a very conservative and at times very backward
organization. I do not agree with Meindert, but he does raise very valid
points. NMEA 2k is better than 0183, but it doesn't hold a candle in
transport capability or flexibility in comparison to Ethernet.

And what capability and flexibility claims are so great as to be useful in
the MARINE industry? Just what about TCP/IP is so useful in this
application?

The world has changed. I am an electronic engineer that has been involved
with both IT and aircraft instrumentation for 40 years. the world has
changed, we need to keep up.

NMEA2K keeps up, and more.

Ethernet and TCP/IP is used by billions world wide. Implementing this
technology allows this "very small" market place you speak about enjoy the
cost advantage of a technology used by the world.

How, exactly? More wire, non-standard connectors (RJ45 in a screw cap?
puh-leeze)

I'm all for cost effective solutions. But, as the saying goes, when all you
have is a hammer, everything looks like a nail.