Accuracy of liquid level measurement using sonar sensor
I've implemented this on my 2 x 23,000 litre water tanks using 2x hc-sr04 sensors mounted on the tank lids and a nano in the pump house. Yes water is a good hard reflective surface to bounce the ultrasonic sound waves off. My tanks have an overflow outlet but not sure how well the eletronic will last? However they are cheap to replace. It is only early days but the overall the results are looking good.
Couple of points:

1.polling occurs hourly. 

2.I only power the ultrasonic sensor just before taking a reading.

3.Has a sound detection sensor to alert on water pump overrun, ie a leak.

good luck