on the boats that i work on, we run the cockpit camera through a vcr that is mounted in the salon. These are pretty big boats, so you may not have the room for a VCR on yours, but its the cheapest way to do it. you might even look into a small 12 volt unit that is commonly used in cars. Be sure that it has a record feature though, many of them are only designed to play tapes, not record them. here is a model you could use.
http://qualitymobilevideo.com/Mercha...uct_Code=VCRT3