Methane gas is obviously lighter than water. But is it lighter than air?
My recollection is that a natural gas leak will sink to the floor -- being heavier than air. Thus, there would be no reason whatsoever for an airplane to lose its lift.
Methane by itself “is” lighter than air, and will thus rise. But when it forms a clathrate with water, the resulting species is heavier than water and will sink. So yes, the theoretical result with airplanes is correct. Melt the clathrate and release the methane and the result is a myriad of bubbles rising through the water (and then the air). In water these bubbles cause the density of the water column to drop much like an fluidized bed causes sand to act like water. So the resulting “aerosolized” (methanolized??) water column will no longer support the weight of boats, which will literally “drop like a rock”.