Here's the response from Airmar:
It sounds like some of the epoxy or urethane that constitutes the acoustic window of the transducer has pulled away. I've not heard of that happening in the way you have described, but since there is a ceramic element physically vibrating within the encapsulation material, perhaps after so much time, the material has become delaminated from the ceramic and the material weakened. It's possible I suppose but impossible to say for sure. I can say that although the transducer may work now, it would probably soon have developed a water path into the internals which would cause the transducer to fail. I think you made a wise choice replacing the transducer when you did. Thank you for the update on your transducer by the way. We take pride in what we build here at our factory in New Hampshire and it's gratifying to know that our product has served its owner well over the years.
I think it's interesting that the ceramic element is physically vibrating within the encapsulation material.