On the Lowrance HDS units there is an area in the settings that allows you to compensate for the location of the transducer.You can either compensate up to the water line [positive number] or down to lowest point on boat [negative]
I have mine offset to the surface. I'm sure other units have this option if you dig through the manual
Its winter,what else do you have to do?
[only time I read the manuals,]
page 81
http://www.lowrance.com/Root/Lowranc...0041-001_w.pdf