Sensors for boat landing guidance

Looking for ideas on what sensors to use to tell the boat driver proper line up for landing into a boat lift station. This needs to be tighter, then GPS can deliver as it needs to be correct within about 4". So i need to identify on center so i can determined whether left or right action would be needed. I would say initial guidance range would be about 20-30 ft out from resting position.

Any thoughts? A pair of transponders at the corners of the lift station?

laser/IR and triangulation?