You could run a simple webserver, and put the NMEA into an HTML file. Then point Safari at that file.
Good solution although it sounds more like client-server than "peer-to-peer". It would probably need to be an active web page rather than simple html if you want the data to be dynamic without having to refresh the page all the time on the iphone.*That's* the best plan - turn the boat PC into a wireless base station (aka peer-to-peer networking), connect to that, and rig up something with whatever IIS might be included in the boat PC's Windows (XP Pro?)
Or try http://www.mochasoft.dk/iphone_vnc.htm to "see" the PC screen & control it on the iPhone
What about the other way around - taking GPS data in NMEA format from the iPhone and using it as an input to a chart plotter (standalone or laptop-based) as a substitute for a seprate GPS receiver? I can see value in that, though I have no idea where to start.