hey - if the thing is entirely underwater - set your IOR for the bubbles to 1.33 and don’t even worry your head about the volume of the water.
IOR can be thought of as a ratio between how much light bends when passing from one material to another, with 1.0 being the reference (air). Think about it - a light ray passes from air into air, that’s 1/1 = 1 no change. A light ray going from air into water has it’s angle changed proportionately 1/1.33. Going from water to air it’s 1.33/1
So, when underwater think of the ratio of air to water in reverse and use the water IOR for the air bubbles. Ie, your air bubble is effectively a “negative” water bubble. The light ray bends the same amount.
Also, set the IOR of the mesh of your water’s surface to 1.33 as well.