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.
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Before finalizing on this I had bought another one which was a waterproof modified version of HC-SR04 from here
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The 1st one is much more rugged and waterproof as compared to the latter one.