I'd go with a standard fuel tank float sensor for a car; can be bought all over the internet relatively cheaply, or if you want to make your own, the basic priciple is a float with a hinged arm acting as a very simple pot:
If you're using a standard 500 litre intermediate bulk container (i.e. a cube) then there's not even a need for any processing - resistance of your pot should be directly proportional to water level.