HC-SR04 ultrasonic sensor water proofing
Sorry for bumping up an old question but I'm in the same boat of requirement.

And to my surprise there are quite a few new sensors in town:

This one has 2 transducers instead of 1 therefore has less minimum detection distance as compared to JSN SR04T. It is also completely sealed and waterproof. 

https://www.ebay.com/itm/253978673298

Before finalizing on this I had bought another one which was a waterproof modified version of HC-SR04 from here

https://www.ebay.com/itm/254058015473

The 1st one is much more rugged and waterproof as compared to the latter one.