My sister had a large ferro cement hulled staysail schooner (well, large to my eyes)
It had a sonar which was mounted in a tube. The sonar head could be lowered down
through the tube to look forward and positioned at various angles.
Much of the time it was raised above the bottom of the tube - which was flush to the
bottom of the boat. This was good as one time the boat hit one of those huge logs which
float under the water and we could hear it rolling as the long keel proceeded over it
(no damage to the ferro cement keel) and since the sonar head was retracted
into the tube (not protruding down) it was not damaged.
One good use was coming in to a harbor/bay down an unmarked channel.
The sonar showed the deep water and we had an easy go of it.
Also, particularly when anchored down it was interesting seeing some rather
large fish below... and being impatient when none seemed to care at all for our bait.
This was in the San Juan Islands and also into Nanaimo and on to Princess Louisa
sadly now in south Texas
not in the glorious NW