I'm open to recommendations
You could try
this one. Inexpensive and waterproof.
I would purchase two of them, one for generating ultrasonic pulses and the other to receive them. Then use a variable-frequency oscillator to find the resonant frequency of the transmitting transducer by sweeping the frequency near 40 kHz and observing the received echo amplitude. Of course the receiving transducer is also resonant, but hopefully this procedure will get you into the "ball park" of what you want the transmitter to do. When you find the maximum amplitude at the receiver transducer, record that frequency and use it when you pulse the transmitter. Gating out about 100 cycles at a frequency close to 40 KHz will give you a two and a half millisecond "chirp" which any dog should be able to hear and locate the reflection of.
You might consider adapting this concept to blind people. You would need two transducers for receivers, located near each ear, and electronics to change the received ultrasonic pulses to audible pulses sent to earbuds. Perhaps this could all be integrated into an "in the ear" device, one for each ear, but the prototype could be mounted on a head-band and worn like stereo head-phones. You might want to make the transmitter hand-held (like a flashlight) for better localization of the reflected ultrasonic pulses.
There is a huge market in prosthetic appliances to aid the handicapped, and 21st Century technology is just beginning to address that need.
Good luck with improving the life of your dog.