Chuck,
Clever question that's hard to answer. There seeems to be a tradeoff between proximity to target and cable length. A long cable means close to target but weak signal. Short cable, like with a hull mounted transducer, means far from target but strong signal. I found when using my 50 m towfish cable I must stay closer than 10 m from the seafloor to get acceptable coverage. When targets are deeper than about 10 m a towfish seems to afford significantly sharper images. At 30 m the targets must be substantial to be seen from the surface. So my rule-of-thumb has become: shallower than 10 m - no need for a towfish, deeper than 10 m - use the towfish (if I search for smaller targets). In wavy conditions a towfish always helps regardless of depth, but then the cable doesn't need to be 50 m!
The images show the same wreck as it looks when using the towfish and when scanned with a hull mounted transducer the same day.
Towfish
Hull mounted transducer