SeaClear II freeware: http://www.sping.com/seaclear (freeware).With MapCal II tool included in SeaClear, you can calibrate scanned maps and convert them to the lighter WCI format. Scan the maps in A4 parts, stitch them with software such as GIMP and reduce size down to 30 percent, because images larger than 15 MB take too long to load in MapCal (level of detail will be more then sufficient). Yahoo Group http://groups.yahoo.com/group/seaclear_mapping.
Use small netbook attached with Velcro straps on navigation table.
Some high-level maps can be obtained from http://openseamap.org. Download from http://sourceforge.net/projects/openseamap/files/Maps.