Sensor For Continuous Water Level Measurement
A bit of googling helped:


a dozen ways to measure fluid level


floats
load cells
magnetic level gauges
capacitance transmitters
magnetostatic level transmitters
ultrasonic level transmitters
laser level transmitters
radar level transmitters

disadvantages of ultrasonic fluid level sensors



  With some liquid media, however, these ultrasonic waves are absorbed rather than reflected. If you’re thinking about using an ultrasonic level sensor, be sure that your liquid medium reflects ultrasonic sound rather than absorbing it.
  
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  Agitated liquids, turbulent liquids, foaming, sloshing, and other activity can hamper the performance of ultrasonic sensors. In applications where liquid agitation is common, ultrasonic sensors may therefore be a poor choice.


Alternatives mentioned: submersible pressure sensors (hydrostatic pressure level sensor), optical liquid level sensors