It is most likely turbulence but 15 MPH is quite slow for it to cr@p out unless it's very deep.
There is normally a bit of adjustment (up...down....angle) that can be tried.
Another trick is to fill the gap between the transom and the leading edge of the transducer with silicon. I'm using a transom type transducer bonded to the inside of the hull (gives a false water temp reading) on a 15 year old Lowrance and I can get a clean trace to about 30 meters at 25kn, breaks up beyond that.