The GPS is pretty accurate but your antenna in the unit puts you the distance from unit to transducer as a difference or error. In essence when you go back to the way point you could be 2 or 3 times that difference from the spot. Get you a external antenna/ GPS receiver and mount it as close as possible to transducer. Next time take a buoy, when you mark structure toss the buoy and mark way point. Go away from it then come back to the way point on unit then check how far you are from buoy. Try this in a couple of different places to give you a idea of what's going on that you can visualize.