Hello,
As an ex-industrial instrumentation technologist, I can confirm that most open tanks to depths up to 4-metres would have used ultrasonic devices. These were very expensive, but very reliable (Pepperl & Fuchs for example). Calibration is complex. I tried cheaper versions, but they are prone to wildlife like spiders taking up residence. Floats and wires and pulleys are still widely used. Have a look at heating oil tank gauges - these seem to be cheap, contactless and wireless - probably ultrasonic. Triangulation with a couple of optical sensors is possible, and the reflective type which use a reflector like a bicycle mudguard have surprisingly long ranges (20-metres or more). I am about to do something for calculating the volume of a rainwater storage tank. I will be looking at a differential pressure sensor, but the mechanical methods might be in the running. Laser distance finders are remarkably accurate, but would need some means of damping the target movement. Also might be worth looking at how cheap digital vernier calipers work - I believe this is capacitive and again it is reliable and cheap - you could look at mechanical scaling down of your range to bring it into the caliper range. I do know that the environment agencies use float methods for bomb-proof river level measurements.
Good luck