I understand the whole reasoning around the motion blur, and I certainly were to agree if it was a slowmo shot. Of course it can also be an aestethic choice, even at full speed. My point was just that it's an easier sell as realism with motion blur , once again only imho.
Nice to hear your reasoning with the bubbles! I'm sure you'll nail it I would also instictly think about a vel field akin to ink in water + upward buoyancy, sounds like a plausible track.
If you advect the bubbles using the vel field, it's worth playing around with the different kinds of advection in the dop node, can give very different results.
Best of luck!