I think that Shellback and Wiskers stated the best fix for what you are seeing. The sepration of the transducer and GPS Receiver can introduce an inaccuracy of up to two times the linear distance between them. That is in addition to the inaccuracy of your GPS Receiver at the time you marked the waypoint as well. Move the GPS Receiver as close to your transducer as you can get it and be sure that it is not mounted where some GPS satellites will be blocked (this may mean mounting it on a stem or VHF antennae mast) for the best possible accuracy.