seafarer transducers were recommended to be in an oil filled tube in the good old days. It was never neccesary as we found when the oil ran out and we put water in instead.
Epoxied in and one good tap to remove and replace if needed
Fine. I've epoxied them myself in the early days. The issue I had was the assertion that silicon would basically cock it up.
Each to their own. On my fishing boats transducer just sat loose in the well and we threw a bucket of water in there each trip. Worked fine. Not neat and tidy but it did the job. A good bodge job
It wont work if there is a void, the sounder needs to be against a surface with the sea touching the other side of the surface. The original pic was a humber, no void usually...at least not intentionally-an unintentional bubble in the construction would be an unlucky problem.