Wideband impedance matching of a power amplifier
If the transducer is  a piezo, it will exhibit strong impedance variation with frequency. There will be no one fixed optimum source impedance other than zero (or as close to zero as you can get). Also, if it is a piezo, you won't be varying the frequency very much. The best drive happens when excited at the resonant frequency. Because of the relatively high impedance, you may need a significantly high drive voltage. Typically a conventional audio power amplifier is not a good driver because they are designed to drive a low impedance (usually 4 ohms to 32 ohms).

Some piezo transducers are resonant above audio. But, if it is for underwater use, the frequency can be as low as 2KHz. You have to know the resonant frequency! If it is for echo-ranging, then transient response is critical. 

This is not a very clear response, but it is very significant challenge to design a "generic piezo driver". Much more specific design guidelines can be provided if you can specify the piezo resonant frequency, the impedance at the resonant frequency, and the desired output power.

Jim Wagner  Oregon Research Electronics