I had an article (Good Old Boat??) about building your own computer onboard. The author had built a computer into the space he had and hence did not the case to worry about. Generally there is a lot of space of a sailboat but it exists in small to medium sized chunks that are often oddly shaped. If you build in the computer you might be able to use some of these spots - works well at a chart table.
Current usage I am not so sure about, but I think the author of the article had found a 12v computer system somewhere so it saved relying on shore power or a genset or inverter