West system provides pretty straightforward instructions. As long as the area of the hull you are mounting the transducer is not cored (solid fiberglass) you can get acceptable readings through the hull. If you are installing a high end sounder for offshore fishing you would not go this route but it will work well for your application.
WEST SYSTEM | Modifying and Customizing Boats - Depth sounder installation
Setting the offset is a matter of preference, do you want the unit to read water under the keel or depth from the surface?, either is fine as long as you know what it is displaying. Measure the depth manually (lead line or long boathook) in your slip and adjust the offset to your preference.