The characteristic acoustic impedance of water is 1.5 MRayls. Piezoelectric ceramic is ~30 MRayls. Without matching, 90% of your electrical power bounces right back into the transducer as heat.
In the pantheon of underwater acoustics literature, few texts carry the quiet, dense authority of L. Stansfield’s Underwater Electroacoustic Transducers . While Urick’s Principles of Underwater Sound is the poet of propagation and Burdic’s work is the strategist of sonar signal processing, Stansfield’s treatise is the materials physicist and the electrical engineer’s bible . underwater electroacoustic transducers stansfield pdf
He explained that water has a tensile strength limit. If you drive a transducer too hard, the negative pressure half-cycle tears the water apart, creating vapor bubbles. These bubbles collapse violently, eroding the transducer face and scattering acoustic energy. The characteristic acoustic impedance of water is 1