Stay away from the propellers
The propulsion propellers is the dominant noise source on most vessels. The noise is transmitted through the sea water, and may often reduce the performance of your system.
For this reason, the transducer must be placed far away from the propellers, which means on the fore part of the hull. Positions outside the direct line of sight from the propellers are favourable.
On small vessels with short distances it is advised to mount the transducer on that side of the keel where the propeller blades move upwards, because the propeller cavitation is strongest on the other side. The cavitation starts most easily when the water flows in the same direction as the propeller blade, and that is to some degree the case at that side of the keel where the propeller blades move downwards.
(Photo: US Navy / Wikipedia)