Is keel off-set programed into depthfinder?? "most" units when calibrated are between 2 and 3 ft off on the shallow end OR... the deep end. I have done some testing for Navico and it is nearly impossible to be exact unless you use a rope to verify your depth and set Manual off-set accordingly.
I'd verify with a rope unless you have someone that knows how to calibrate a transducer, or do both at the same time.