Mount the transducer at a safe distance from bow thruster(s)
Bow thruster propellers are extremely noisy. When you decide where to place the transducer, you must consider the noise created by most bow thrusters.
When in operation, the noise and cavitation bubbles created by the thruster may make your ES18 Split-beam transducer useless, almost no matter where the transducer is installed. When the bow thrusters are not in operation, the tunnel creates turbulence. If your vessel is pitching, the tunnel may be filled with air or aerated water in the upper position and release this in the lower position.
In general, the transducer should therefore be placed well away from the bow thruster(s).
However, this is not an invariable rule. Certain thruster designs - combined with their physical locations on the hull - may still offer a suitable location for the transducer, even close to the thruster. If you are in doubt, consult a naval architect.