I would not advise your plan
The Raymarine instruments are on their own power bus (or should be). Any 12V item that you plug in would draw current and cause a voltage drop for the instruments, and could potentially cause electrical noise on the system. For example, my instrument bus supplies power to all of the nav instruments and also to the autopilot, but the motor on the autopilot is switched from a separate breaker and is not on the instrument bus.I like to keep everything separate. If you are thinking of plugging in a spotlight or something else that draws a lot of current, you are much better off running a dedicated line to the helm and drawing power from a dedicated breaker on your panel.Running a separate line is not as difficult as you might think. You probably need a wire fishtape and a few hours, but it is quite do-able.