Cheap sensor for measureing air dissolved in water?
Deep ends on % aeration you can get, but:

Density will fall as aeration % rises. Easily measured with eg a float.
A driven vibrating element would be affected by density - say a piezo buzzer element.

You MAY be able to use the paramagnetic properties of Oxygen. This can be used to measure O2% in air. How swamped it is by water I know not.

Sound transmission properties will alter. Whether usefully is TBD.


Density sounds the easiest.