I've got a Lowe 1848 jon w/30 HP Evinrude jet and have a Garmin 240 w/transom mounted transducer and can read depth at WOT. Granted, on my rig, speed is only 22 MPH, at WOT and yours may be much faster but the bottom of my boat is pretty beat up and I still get a reading. Is it possible you've got the jet mounted a bit too low and that's where the turbulence is coming from? The leading edge of my pump is about 1/4" below the hull. I did install a transom mounting board before mounting the transducer so I could move it if necessary but guess I got lucky the first time.
walleye1