Hi DS
I have heard of guys hacking these to take the bluechart maps. However they have been having problems with the 200w showing the contour lines while on the move i.e. you can't see how deep the water is.
If you buy the Nuvi 500 then bluechart is supported off the shelf. If you are wanting to get chartplotting ability at a reasonable price have a look at the GPSMAP 76 or 60 range.
Cheers
Brent