If you were to include a "Marine" attribute, populated with "Y", making a Garmin marine chart that runs on any Garmin Marine unit (gpsmap 4xx, 5xx etc) is dead simple. You could actually do it directly from RM, using cgpsmapper in exactly the same way as the IMC.
This will make use of the "Type" field.
It would mean that your exported shapefiles could also be pulled straight into GME, the charts can be played with by adding buoyage, POIs etc, then exported directly using cgpsmapper.
A huge number of the waterborne fishing public use Garmin chartplotters, so it would be a shame to leave them out when making bathy charts for them is now so simple.
This now makes
bottom hardness charts a reality on Garmin chartplotters, but you'll need to give me a while to play with it. I'll put a tutorial up on my website when I've got it in order, but the chart export itself works perfectly already.
T