Well you, of course, know that Datamarine and the Commodore computer manufacturers share something in common.

There was a group in Massachusetts that still provided parts and repairs. DMI Marine, Inc. www.dmimarine.com/parts/index.html

It's possible that you have a bad cable, or unit board, but I susspect the sender.

Here's a manual for (I'm not sure it's your unit): http://www.sailingseadragon.com/Manuals/DatamarineS200DL.pdf

http://www.ess-kayyards.com/catalog/page11.html

You could try EBay -- there are a number of these items.