hi nau,
As you are planning to use the Piezo transducer from a printer head as a water depth measurement transducer, my strongest advice would be to determine how you intend to mount the transducer and carry out shallow water tests using the Amp to directly drive the transducer, you may have to tune the cable/transducer to get resonance at 33kHz.
This should give you some empirical data regarding the transducers performance.
Do you have any data on the beam width/pattern you are likely to get thru water.? for a 33kHz its typically around 15deg, 22degs.
The dual 210/30kHz transducers [ 1kW] I used in my echo sounder products, cost around £1500GBP.
Eric