i havent' found an inexpensive plotter/chart solution for the bahamas, portable or else...
the best set up is OpenCPN with NOAA charts for the US and Arnis NV charts (CD / chart books) for the bahamas.
http://www.nv-charts.com/
they run about $80 per region (near bahamas, central, and south) and are very accurate but they have a 3 region special for 149. The charts install easily for use with OpenCPN.
i've used them extensively last winter all the way to Georgetown and was my primary nav system since i didn't have the Cmaps for the furunos.
www.opencpn.org is free (open source) and so are the NOAA charts. it is a very reliable program and works with any inexpensive USB or bluetooth gps. I've used it for the past couple of years now, from Cape Cod all the way to Houston and on many trips to the Bahamas, as reliable and accurate as built in plotters and uses fewer system resources on your computer than commercial software.