Howdy Drewski,
It looks like we are creating a workable idea here

.
As each of us makes a suggestion, the other sees variations that may lead to a simple answer.
One occurs to me right now, and it's based on your suggestion of a speedometer for boats.
What if we put a small anemometer in the bubble stream?
Normally, an anemometer is placed horizontally - to measure wind speed.
If we had a small one that was operating on a vertical axis instead, the bubbles would become the prime mover for it.
Using this method, it should be possible for the device to register even tiny gas production rates, as the bubbles will collect in the cups of the rotor.
The bearing for this device can be simple too. Just a stainless steel wire passing through a hole in a plastic block. The plastic block would have 4 to 6 plastic spoons arranged around the periphery.
Bubbles would be directed to it, by careful placement of the gas feed tube.
By using a fairly small diameter at the outlet of the feed tube, the bubbles would be smaller, and would be more inclined to hit the spoons, instead of bypassing them.
An optical device could then be set up to count the turns of the 'water wheel' - - errrr 'bubble wheel'
This could lead to the invention of 'bubble memory'
Hmmm, looks like a project for my machine shop
Best wishes,
Steve.