NMEA 2000 is a closed CAN-BUS standard for marine electronics. It would be awesome to have a shield that can accept the standard NMEA 2000 or that has a screw down terminal to accept bare wires.
NMEA 2000 supplies both power and data in it's cable. Not sure if it's adequate to power the ardunio. http://en.wikipedia.org/wiki/NMEA_2000
If someone out there can handle the hardware and board side of things, I could help do the research.