I have been looking at ultrasonic transducers like the ones here: http://www.futurlec.com/Ultrasonic_Sensors.shtml but even the waterproof ones typically have something like this in the datasheet:
... do not use this sensor in the following, or similar conditions. a) In strong shock or vibration. b) In high temperature and humidity for a long time. c) In corrosive gases or sea breeze.
I am not looking for anything too sophisticated; navigation is by command and/or GPS. Obstruction detection is mostly a safety feature (try not to hit anything at full speed). I think a couple of wide cone with overlap in front (so if I detect something roughly the same decreasing distance away in both, it is likely in front of me) would work.