I tried out the $35 fishfinder/depth sounder in the ocean this weekend, and it works quite well. 100 meters range as advertised, matching a commercial sounder.
The heavy, durable transducer is connected to the fishfinder by a removable cable with waterproof barrel jack and plug, so I inserted a matching plug/jack pair and hooked up a scope.
The open circuit driving signal is 400Vpp, 200 kHz, 100 microseconds duration. My cap meter reports the transducer capacitance as 2.9 nF at 1 kHz excitation frequency. The transducer is probably resonant at 200 kHz, so that capacitance is not a good guide for the load impedance, but no wonder that battery life is short.
Unfortunately, the snap-together clamshell case of the fishfinder is very well designed and solid, with no screws, and will have to be cut apart.
More later, but my preliminary conclusion is that the purchase is worth the price of the transducer alone, as I haven’t seen even naked transducers for less than $35 on Alibaba.