Here is Datamarine's reply.
Back then the transducer assembly consisted of a crystal glued inside a
plastic cup, inserted in the housing and filled with epoxy. The water is
seeping between the glue holding the cup inside the housing and finding
it壮 way to the top of the epoxy. It is not the way you mounted it, is all
internal to the transducer.
Seems for leakage to occur then the glue must be in direct contact with the water?? I read this as saying that the glue is inside a plastic cup and for leakage to be happening then the plastic must be failing allowing the water to come into contact with the glue?? Pus I'm not sure how the epoxy get added and if that is an additional barrier to the water.
I submitted my question both to Raymarine and to airmar about transducer deterioation with age. I'll post if I get a good answer.