If you have an I/O there might be a hole or port in the leading edge of the
bullet shaped lower leg that has a thin hose going to the guage. If so it
could be blocked by sea weed, etc right at the entry point. You also could
take off the hose from the back of the guage and blow back thru it with some
compressed air. Of course you could also have a paddle wheel pick up on your
depth finder transducer which electrically actuates the guage.