Just be aware and ensure we are talking the same language.
"Thru" hull as the yanks like to call it means a hole in the hull and the transducer is on the underside of the boat. A true skin fitting.
Transom mount is as we all know, the standard transducer that comes with many units, for external mounting on the transom.
Here, through hull, tends to mean, a transom mount transducer sending and recieving through the hull material (grp etc). Often mounted on a bed of silicon, plasticine, epoxy or in an oil filled reservoir.
Hope that helps,
Rob