hi Ayne,
Running at around 20kHz, it should send all the local dogs quite 'barking mad' after a few minutes of operation.
Some dog barking deterrents work around these frequencies
There's a guy in the UK, developing a sound system, I think its around 24kHz, that is used near shopping centres in the evening, to 'disturb' and drive away gangs of youths who sometimes gather in these areas.
It seems to work, so you may drive away all the young ladies!!
I expect you don't require continous water level monitoring, so you could just have a 1 Sec pulse burst every 10 minutes or so.
Regards
EricG
edit:
As Oznog points out, the overall resolution would be low at 20kHz.
G8RPI
When the lower frequencies are used a Sonar, the velocity of sound in seawater is around 1500m/sec compared to 330m/Sec
in air, so you get a 4.5 increase in 'apparent' resolution.