How to package an ultrasonic transducer so it can be submerged?
I am going to distill my three comments into an answer. This may not be the answer to your question, but perhaps it will help. But first I must provide a disclaimer: I am not an underwater acoustics expert. I worked in the field years ago designing electronics that interfaced with transducers and hydrophones. With known TCR and TVR curves (Transducer Current and Voltage Response: measured in open water by professionals) all of the specifications for how much current to drive in order to achieve the desired sound pressure levels at some range over a set of frequencies were handled by a senior acoustic scientists. That work was non-trivial.

With all of that said: This is more of a materials science and physics question than electrical engineering.

Most of the transducers I have used in the past that needed to be submerged for long periods of time at full ocean depth have generally been potted. The potting compounds themselves have been proprietary and very specially designed. So I can't even begin to comment on how you should mix one one. I suppose it depends a lot on your application too. But perhaps this paper submitted to IEEE Oceans about building a low cost acoustic modem can spur some ideas, or maybe some of the references could be of help: http://cseweb.ucsd.edu/~kastner/papers/oceans10-low_cost_modem.pdf

The other method I've seen used is floating the transducer in some type of mineral oil with a thin membrane. But I've never seen this used for a transducer going down to the ocean floor though. I am not sure if something makes this approach unsuitable for that type of application or not. This might be an easier for you acoustically because the oil will behave more similarly to sea water acoustically* (Some oils are denser than others - but their density will not change much with pressure and temperature - so you can cal this out of your system). But there are two issues you will run into with this:


Your package will be more brittle. Your housing may or may not be rigid. But regardless you will need to be careful not to puncture or break the housing. You may also have to worry about end cap(s) that need to be sealed up and a water proof connector for your cable.
If you use PVC or various other materials you will need to consider water absorption. Over time PVC, Polyethylene, Teflon, etc, will absorb water and your oil will get watered down. You will need to worry about this shorting out your transducer - especially if you are deploying in sea water.


If you are not planning on putting this in the water for years on end, item #2 may not be a big deal.

Regardless of what you do - PROTECT THE OUTPUTS TO THE TRANSDUCER FROM A DEAD OR PARTIAL SHORT WITH SOMETHING!! Fuse the power rails (or use a PTC), and buffer your signal lines. Or, if your amplifier is built into your electronics package and you are just running a high current waveform directly into the transducer still consider some way of fusing those lines. Short circuits in this sort of environment are things you should expect. I've seen many DC/DC converters and ADCs burn up because of shorted sensors - sensors that shorted after 3-4 years of underwater deployment. They just gradually began to draw more and more current and baked the DC/DCs.