I've been obtaining good results by modulating the current to the microphone -- on / off / on / off. ...
I'd love to have you weigh in on why this may be working.
Congratulations, you have re-invented "chopping"!
That was a technique used back a generation or two ago to handle DC values with circuits that otherwise worked only with AC signals. By turning the microphone on and off, and as long as you can "calibrate" the results, then you can use that method as a crude pressure detector. You may get even better results sealing the back of the electret capsule (as with RTV from a squeeze-tube) .
They didn't cover this method in those 'basic' videos you suggested. But hey, if you're an expert,
Mr. Ford mentioned putting a small hole in the rear of the capsule to equalize the pressure. He didn't make a big deal out of it, so it was probably easy to miss.. By sealing up this hole, the capsule becomes more sensitive to absolute pressure. However, the diaphragm itself is still capacitively coupled to the internal amplifier, so it is not capable of sensing a steady-state pressure.
And as far as the tubes are concerned. The water/air/transducer interface that you're suggesting is horrible for measurement integrity and stability. You really need a 100% water/transducer interface that's been completely purged of air bubbles for good data.
It works in many cases. But not all cases. Since there are dozens of factors you didn't reveal here, we would have no way of knowing whether a specific solution would work in your (undefined) situation.