I looked at the canopy idea and it didn't appear physically possible without sustained power of God holding the canopy in position high in the atmosphere. I read when I looked the idea up that it was a sheet of water as a shell surrounding the earth.
Probably what happened is much different. When solar eruptions occur over time and in large amounts of proton-electron pairs from the fusion reaction of our sun which is hydrogen are directed towards the earth they might promote the production of H2O in the atmosphere or surface of the earth. And that might form a eventual mist in the atmosphere of water. The bible indicates that before the flood the dew watered the plant life upon the earth. So there is testimony from God that it was very misty.
Mist or humidity in the atmosphere can occur in high concentrations under certain conditions, one of which is a heated condition of the atmosphere.
"Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer." Discussion on Humidity
That said there are other conditions in physics that can cause higher water concentrations held in the atmosphere. A study of precipitation is useful in understand how water vapor is released from the atmosphere as well.
I am imagining clouds prior to the flood that are even at higher altitudes than what we see today. A cloud is a higher humidity phenomena. Which is why it can obscure the sun light.
And although I haven't given the flood and misty conditions prior much consideration, there are other factors such as pressure of atmosphere and more advanced ideas that could have played a roll in the initiation of a global flood.