What type hull? Fiberglass hulls pass signals well and you can glass
the head to the hull directly, or epoxy a pipe mount to submerge the
sounder in oil. You can try the transducer location in a baggie of
water held against the hull on the inside.

Wood hulls need a window made of material that will pass the ping, or
an external mounting.

The location must avoid air bubbles at speed and at rest.

Terry K