How high tech?
This system started out Arduino based, moved up to 1284P processor for the extra SRAM and IO.
Member skyjumper did all the coding for it, it was one of my first Arduino related PCB designs back in Dec '10/Jan '11. http://www.ontargetmarine.com/ RPN4
My only concern with boat autopilots is whether the user is going for an autopilot or completely autonomous. The later is dangerous if your plan is to let it sail around on its own. However if you are just being lazy, then as long asyou have some easy to reach e-stops, then I imagine it will be somewhat do-able.
SunnyJ:
Plan is to sub out the compass and go with a Fluxgate or similar.
If the "heading" output from a GPS won't work for you (the boat needs to be moving to get a heading output) then you want a magnetometer, a HMC5883 or similar. Sometimes you can find GPS + magnetometer combination modules as well.