How about some visual system similar to the helicopter's, but based on visual cues from three shore based light sources.
Three light sources would be the equivalent of, say, red, green and blue, but in the form of pulse modulated infra-red beams to avoid ambient light interference. You need three points of reference to avoid the abiguity of just two. The beams would have to be wide-angled for good coverage.
The boat would have a rotating mirror (much like a radar antenna) viewing its surroundings via a slit or collimator. The mirror would reflect the light down (same as a radar wave-guide) to a stationary detector (avoids slip rings for the electrical signal) which would send a pulse to a PIC processor.
The PIC would have knowledge of the relative location between the "red", "green" and "blue" lights and the current angle of the rotating mirror relative to the boat. There would be enough geometric data to work out the boat's exact position and heading relative to the three lights. Knowing the current and desired positions, the PIC would then be used to produce an error value for the control system to manipulate the directional motors.
If the beams are broken then the control system would have to retain and use the last good reading based on calculating the average position.
Anyone good at geometry?
Just a thought.
See Wikipedia
http://en.wikipedia.org/wiki/Range_lights for a light house version.
Ian