Well you can use differential GPS to get that accuracy up to better than 1m. There are probably land based beacons by a lot of large bodies of water to provide an additional GPS source.

Still by the bank is better than a kilometer away!

Cost wise? GPS modules are about £30-£40. I am using a mere 8 bit processor in my prototype and have yet to max it out. This should leave the basic navigation system done for around the £50 mark. However I think if it were a commercial system you would need to add some gyro sensors to provide an additional approximation for direction whilst steering. They are expensive.