but, as per your suggestion, I went to the West in Midtown Manhattan and played with the 78SC.
You are correct (that's the painful part). Not a chartplotter. Frankly, I think Garmin's advertisements for this
unit are deceptive: on many sites you can see a pic of the unit with a detailed chart on the screen which looks
just like (but smaller than) the screen you see on their chartplotters. As I found out this is not at all the case.

Oh well back to the drawing board. Yes, the 640 is expensive and - like you - I'm not convinced
of the advisability of a touchscreen on the water. It's true it has a rechargeable battery - but
if you want a spare that's another $100. Also, some of the reviews I just read for this unit are
not particularly favorable....

Jerry