Here is Datamarine's reply.
The water is seeping through cracks in the adhesive of the transducer, not
the plastic itself. This will also interfere with the operation of the
transducer since it will corrode the contacts inside. Your transducer is
most likely the DX-111 (or A-111 depending on the age). The replacement is
the ADX-210($160) and has a 2" thread diameter so it will fit in the same
hole left when you remove your existing transducer. You can see this
transducer on our web page listed below under the PARTS page.
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.
My thanks to Steve and Datamarine for his prompt help. Now I understand where the salt water is coming from. Time for a replacement.