KP:

Since you handle vector maps I wonder if you have looked at mapping nautical vector charts.

NOAA publish free vector charts for US waters: "NOAA ENC®s are in the International Hydrographic Office (IHO) S-57 international exchange format."

Most countries now publish similar vector charts, sadly most of them are only available commercially.

See: http://www.nauticalcharts.noaa.gov/mcd/enc/index.htm

FWIW. I don't know of any Linux-based chart viewer, so such a thing would be a welcome addition. If I am wrong about this then I would be happy to stand corrected.