Now you build your own ultrasonic bumpers. Mine just arrived today with lots of other goodies, won't have a chance to test for a while. 5V, single transducer, same pinout as a 4-pin HC-SR04 (those are just $1.15 now
I'll report back after I test accuracy, and if it works in 1-pin mode (like a PING). And waterproofness.
Of course, the waterproof one costs over ten times what the cheapie does. Depending on how long the cheapie lasts, you could let it fail and replace it ten times and still come out ahead.