Drake

The theory is there is no risk of an air gap. The reason we use goop isn't to keep it in place, rather to ensure there is a constant medium for the waves to travel. If waves travel over a gap they will encounter "defraction". The severity of the defraction is dependent upon the size of the gap vs the wave length.

Waves also travel differently through Air, Water and Solid. Going Air->Solid->Water is more distorting than Water->Solid->Water. That is why some of us actually use water in our hulls instead of goop. A "wet mount" we call it. The speed of sound through sea water is approx 1500m/s and through silicone 1480m/s. They are nearly identical. That is why silicon goop is preferred as an adhesive. However, you run the risk of air bubbles with using silicone.

Sound travels through air at only 332m/s. Reduced by a factor of around 4.5? So these air bubbles might seem insignificant, but the affect on the behavior of a wave is actually quite dramatic.

The wave length of 200khz through water is aprox 8mm and through air 1.7mm. So you can see a significant change.

The distortion between solid and water is minimal because the velocity of the wave isnt that different depending on the solid, So their wavelength is closer in value

Hope this makes sense