Even though I am a Mac guy I keep an eye on the dark side and recently found out about Island Time PC, a company that assembles small form-factor PCs for marine installations. Might be worth checking out. For OS, there is Linux and for Windows, I hear Microsoft has finally got it right with Windows 7. On the Mac side, there is the Mini, of course. With either of these you have to figure out what to do about a monitor -- so, often easier to just go with a laptop. Still, I was glad to hear of these guys from a satisfied trawler owner. One tip from the same source: buy a fanless unit to help keep salt air away from components.
Warren
Warren