not sure I can help a lot here but what is the air draft of a boat, the part of the boat above water? The transducer would be a aftermarket install, right? I looked up the GPS 182 plotter and it looks to come with an antenna, no transducer? Could that be right, it just tracks your location and water depth would be separate? Does not make sense to me but I don't know chart plotters.
Oh and welcome aboard, someone with more useful information will be along shortly.